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Reviews in Nanoscience and NanotechnologyVol. 4, pp. 26–49, 2015(www.aspbs.com/rnn)

Fabrication of Nanoscale Cesium Iodide (CsI)Scintillators for High-Energy Radiation DetectionChih Yuan Chen1, Chien Wan Hun2, Shih-Fan Chen3, Chien Chon Chen1,∗, Jin Shyong Lin4,Shardai S. Johnson5, Niyoyankunze Noel5, Niyogushima Juliely5, and Zhiping Luo5,∗

1Department of Energy Engineering, National United University, Miaoli 36003, Taiwan2Department of Mechanical Engineering, National United University, Miaoli 36003, Taiwan3Department of Materials and Minerals Resources Engineering, National Taipei University of Technology,

Taipei 106, Taiwan4Department of Mechanical Engineering, National Chin-Yi University of Technology, Taichung 411, Taiwan5Department of Chemistry and Physics, Fayetteville State University, Fayetteville, NC 28301, USA

The CsI-based scintillators have been widely used for X- or gamma-ray detections. It has been known thatcolumnar scintillators favor the detections with higher efficiency and spatial resolution. In this paper, we report

a facile and low-cost method to fabricate submicron nanoscale CsI columns. We integrated the thermodynamic

calculations, positive chamber design, mold design, electrochemical bath design, anodization process, andsolidification techniques to fabricate such submicron CsI columns. When the CsI melt is confined to AAO

channels with a high aspect ratio, stable CsI columns solidified with smooth surfaces, and free of dendrites

or grain boundaries. Different solidification approaches have been attempted, including mechanical injection invacuum under a negative pressure, injection at 1 atm, 3 atm and 25 atm using Ar gas only, and mechanical

injection with 1.6 atm Ar pressure. The mechanical injection or high-pressure Ar injection at 25 atm producedgood filling of CsI melt into AAO channels. Such CsI columns inside the smooth AAO walls enable the high-

energy radiation detection with high light emission for pixelated scintillator applications.

KEYWORDS: Cesium Iodide (CsI), Nano, Scintillator, Thermodynamics, X-ray Radiation Detection.

CONTENTS1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2. CsI Scintillator Fabrication . . . . . . . . . . . . . . . . . . . . . . 29

3. Research on Scintillators at Nanoscale . . . . . . . . . . . . . . . 30

4. Thermodynamic Calculation of Cs–I–O System . . . . . . . . . 30

5. Experimental Procedures . . . . . . . . . . . . . . . . . . . . . . . . 35

6. Electrochemical Bath and Mold Designs . . . . . . . . . . . . . . 36

7. Anodic Aluminum Oxide (AAO) Template . . . . . . . . . . . . 38

8. Columnar CsI Fabrication by Negative Pressure Injection . . . 41

9. Columnar CsI Crystal Fabrication by Positive Pressure . . . . . 44

10. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . 47

1. INTRODUCTION

In the electromagnetic spectrum, gamma-(�-) and X-rays,

with short wavelengths, are highly energetic than visible

lights. They can penetrate objects for certain distances,

causing atomic ionization and breaking chemical bonds

∗Authors to whom correspondence should be addressed.

Emails: [email protected], [email protected]

Received: 22 March 2015

Accepted: 10 April 2015

of organic molecules. Detecting such a high-energy radi-ation is highly demanded. Since the discovery of X-raysby Röntgen in 1895, X-rays have been widely appliedin scientific research experiments, industrial inspections,radiography, and security and safety monitoring. Radia-tion detection has been a rather long-time research topicthat is still under development.1 The following methodshave been used for the X-ray detection:(1) Traditional method to record the radiation using silver

films (radiography), which requires chemicals and con-sumables, causing not only high cost but also wastes tothe environment.1

(2) Scintillator crystals (in-direct detection): when excitedby the radiation energy, a scintillator can generate visiblelight that can be used for detection using devices, suchas photomultiplier tube (PMT), photodiode (PD), comple-mentary metal oxide semiconductor (CMOS), or charge-coupled device (CCD), and thereby enabling real timedigital imaging.2�3

(3) Semiconductor photodetectors (direct detection):X-ray photons are converted to electron–hole pairs in thesemiconductor photodetector and are collected to detectthe X-rays.4–7 Silicon or germanium doped with lithium,

26 Rev. Nanosci. Nanotechnol. 2015, Vol. 4, No. 1 2157-9369/2015/4/026/024 doi:10.1166/rnn.2015.1061



Chen et al. Fabrication of Nanoscale Cesium Iodide (CsI) Scintillators for High-Energy Radiation Detection

Chih Yuan Chen is an Assistant Professor of Department of Energy Engineering, NationalUnited University, Taiwan. He received his M.S. and Ph.D. degrees from National Taiwan

University, Taiwan, in 2002 and 2006, respectively. He is also a senior patent examiner of

Taiwan Intellectual Property Office and examines over 100 patents per year, mostly in thefields of metallic and ceramic materials. His research interests include phase transformation,

alloy processing, thermal stability of carbide and nitride, nanowires, and fabrication of

metal oxide templates. He has published over 30 peer-reviewed articles and obtained severalpatents for inventions in the fields of steel processing, alumina template, and CsI single

crystal fabrication.

Chien Wan Hun is an Assistant Professor of nano-casting, Department of MechanicalEngineering, National United University, Taiwan. He received his Bachelor’s degree from

Feng Chia University in Taiwan in 1989. He was a lecturer between 1993–2013. His research

interests include mechanism design, machining, and designation of experiment as needed.He has over 30-year experiences in the operation of lathe and milling machines. He is

currently developing advanced vacuum and positive pressure chamber to assist the novel

nanomaterial fabrication.

Shih-Fan Chen received his M.S. and Ph.D. degrees in Materials Science and Technologyfrom National Tohoku University, Japan, in 1976 and 1980, respectively. After receiving his

Ph.D. degree, he spent three years conducting postdoctoral research at Massachusetts Insti-

tute Technology, where he studied casting technology and novel casting process with Pro-fessor M.C. Flemings. Currently he is an Associate Professor at National Taipei University

of Technology. He has been developing advanced nanotechnology for nanowire fabrication.

He has published several papers in the fields of alloy processing.

Chien Chon Chen is a Professor of Materials and Energy, Department of Energy Engineer-

ing, National United University, Taiwan. He received his Ph.D. degree from National ChiaoTung University in Taiwan in 2005. With the support from the Taiwan government and Texas

A&M University, USA, he studied and worked in Texas A&M University for several years.He was an Assistant Professor between 2008–2010, and Associate Professor 2010–2014.

His research interests include thin films, electrochemical mold design, solar cells, and novel

nanomaterials, and thermodynamic computations. He has over ten-year experiences in theanodization and nanomaterials fabrications, with expertise in the research related to alumina

nanotemplate and titania nanotube, nanowires and nanospheres. He is currently developing

advanced materials for X-ray scintillator applications. He has published over 100 scientificpapers and gained patents for inventions in the fields of electrochemistry, thermodynamics,

and mold designs. Because of his active scientific research, he was recorded in the 32th Who’s Who in the World 2015.

Jin Shyong Lin is an Associate Professor of Machining, Department of Mechanical Engi-

neering, National Chin Yi University, Taiwan. He received his Ph.D. degree from National

Chiao Tung University in Taiwan in 2008. His research interests include anodization, casting,electroplating and metal nanowire fabrication. He is currently developing electrochemical

bath, cooling system, and anodization mold to assist a novel nanomaterial fabrication.

Rev. Nanosci. Nanotechnol., 4, 26–49, 2015 27



Fabrication of Nanoscale Cesium Iodide (CsI) Scintillators for High-Energy Radiation Detection Chen et al.

Shardai S. Johnson is an undergraduate student at Fayetteville State University. She is amajor in Chemistry and a minor in Materials Science. She conducts research on nanoma-

terials and characterization by electron microscopy. She is currently supported by National

Science Foundation.

Niyoyankunze Noel graduated at Fayetteville State University in December 2014, with aB.S. degree in Chemistry. At FSU he conducted research in nanomaterials and characteri-

zation by electron microscopy. He was supported by FSU-RISE program.

Niyogushima Juliely graduated at Fayetteville State University in May 2014, with a B.S.degree in Chemistry. His research at FSU was on nanomaterials and characterization by

electron microscopy. He was supported by FSU-RISE program.

Zhiping Luo is an Associate Professor of Physics and Materials Science, Department of

Chemistry and Physics, Fayetteville State University. After receiving his Ph.D. from ChineseAeronautical Establishment in 1994, he worked as a Principle Investigator at Beijing Insti-

tute of Aeronautical Materials, China. From 1996–1997, he spent two years in OkayamaUniversity of Science, Japan, as a postdoctoral researcher to study electron microscopy with

Professor Hatsujiro Hashimoto. In 1998, he moved to Materials Science Division, Argonne

National Laboratory, as a Visiting Scholar and was then promoted to Assistant Scientist inFebruary 2001. Between 2001–2012 he worked at Texas A&M University as a Research

Scientist of the Microscopy and Imaging Center, and a Graduate Faculty Member of Depart-

ment of Materials Science and Engineering. At FSU, his current research focuses on thenovel nanostructured materials with enhanced properties, with support from NSF and DoD.

He has authored over 200 articles in peer-reviewed journals.

Si(Li) and Ge(Li), respectively, are widely used in analyt-

ical instruments of X-ray energy-dispersive spectrometer

(EDS).

(4) Gas-filled detector: when X-rays travel in a gas, gas

molecules are ionized, producing positive ions and free

electrons.8 If an electric field is applied in the gas cham-

ber, these ions and electrons are separated towards oppo-

site directions which can be detected in a current. This

method is widely used for dose measurement, such as gas

detectors. It is also used in the analytical instrument of

X-ray energy-wavelength spectrometer (WDS).

Among these methods, since scintillator method can be

associated with digital imaging and storage, it is replac-

ing the traditional radiography. This paper deals with the

fabrication of scintillators at the nanoscale, focusing on

the CsI-based scintillators due to their unique high per-

formance and easy manufacturing. When doped with thal-

lium (Tl), CsI(Tl) crystal scintillator is one of the brightest

28 Rev. Nanosci. Nanotechnol., 4, 26–49, 2015




scintillators, with maximum wavelength of light emission

around 550 nm, well suited for photodiode readout. It

has been widely used for detecting X- and �-rays. TheCsI(Tl) scintillator converts incident X-rays into visible

light with a very high conversion efficiency of 64,000

optical photons/MeV.9 On the other hand, sodium-doped

scintillator CsI(Na) is a widely used material nowadays,

with light emission in the blue spectral region which

well matches the photocathode sensitivity of bialkali pho-

tomultiplier. It exhibited fast light in alpha scintillation,

which is a unique property that was not found in other

alkali halides, such as CsI(Tl), NaI(Tl) or pure CsI.10

Recent, CsI(Na) was found to be a promising detect-

ing material for scientific exploration of dark matters in

astronomy.11

In this review, at first the CsI scintillator preparation

methods are outlined, and the effect of nano scaling on

the X-ray luminescence is discussed. To guide the sample

fabrication, thermodynamic computations of the Cs–I–O

system are conducted, followed with experimental prepa-

ration of CsI nanowires, including aspects of electrochem-

ical bath and mold designs, large anodic aluminum oxide

(AAO) template fabrication, and columnar CsI fabrications

by negative and positive pressures.

2. CsI SCINTILLATOR FABRICATION

To date, the CsI-based scintillators have been prepared in

the following ways as the detectors for X- or �-rays:(1) Bulk crystals made by Bridgman-Stockbarger or other

method.3�12–14 Recently, bulk CsI(Na) and CsI(Tl) crystals

samples have been grown by the Bridgman-Stockbarger

method.13�14

(2) Traditional continuous thin foils by various deposition

methods from solutions, typically containing plastic scin-

tillator dissolved in toluene or xylene. The thin scintillator

foils could be obtained by solvent dissolution of the scin-

tillator solution,15�16 evaporating the scintillator solution in

vacuum,17 or spinning the scintillator solution on a rotat-

ing plate to obtain uniform foils.18–20 The CsI foils were

generally deposited on Al substrate, with a large thickness

range from 2–500 �m.

(3) Vertically aligned needle or micro-columnar structures

by various vapor deposition methods.21–32 Vieux and Groot

described the preparation of needles from alveolate sur-

face of aluminum substrate, and such alveolate surface

was prepared by anodization in an electrochemical bath.23

The anodization resulted AAO nanopores (nanotubes) on

the surface, which facilitated the growth of thinner nee-

dles. Nagarkar et al. reported micro-columnar structures

resulted from vapor deposition, with thickness ranging

from 30–2000 �m and up to 15× 15 cm2 in area.25�27�30

Such vertically grown structures reduce the lateral spread-

ing of light, and therefore improve the detector spatial

resolution.30

(4) Scintillators in silicon microcavities, microchannels

or wells with light guides.2�33–42 In a report by Rocha

et al., a deep reactive-ion etching (DRIE) technique was

used to achieve perfect vertical side-walls with 515 �m

depth and 100 �m pixel square size.35 The inner wall of

the well that were coated with aluminum for improving

the number of photons which arrives to the detector and

for reducing cross-talk between adjacent wells. Simula-

tions and modeling showed an improvement of approxi-

mately 26% in the detection efficiency using an aluminum

layer.38

The use of light guides to scintillators in microchannels,

microcavities or wells not only improves spatial resolution,

but also the detecting efficiency.2 As shown in Figure 1,

in the case of continuous planar scintillating foil, the spa-

tial resolution is low, which is about on the same order

as the foil thickness (several hundreds of micrometers),

due to the lateral spreading of the lights (Fig. 1(a)), while

in the case of scintillator confined in narrowed channels

with light guides, the lights are restricted to limited areas,

with spatial resolution similar to the size of the scintillat-

ing crystals with significant improvement (Fig. 1(b)). In

addition, note that in the case of low dose weak X-ray

radiation, the signals on the photodetector by the planar

scintillating foil spread over a larger area, resulting signals

with lower signal to noise ratio (SNR) (Fig. 1(a)) which

are difficult to be detected; while the columnar scintilla-

tors with light guides give signals with a higher SNR that

benefits to the detection with higher sensitivity. As pointed

out by Badel et al., although the vertically grown colum-

nar scintillators without light guides could improve their

spatial resolution, they showed low performance in terms

of SNR and sensitivity.36 The usage of light guides in the

radiation detection is desirable.

However, there are several challenges associated with

the current developments of these scintillator detectors:

(1) The cost to produce Si microchannels is high, which

involve multiple procedures, including cleaning of the sub-

strate and placement of OmniCoat; deposition of SU-8;

soft bake; placement of mask and exposition to ultraviolet

light; post exposure bake; development; washing and dry-

ing; placement of aluminum; removal of SU-8; placement

of the scintillator inside the cavities; polishing of the top

surface; placement of a final aluminum layer in the top of

the detector.2

(2) The current method produces the channel size in the

order of several micrometers or larger, and up to date

there are very limited reports on smaller channels under

micrometer due to the fabrication challenges.

Therefore, in this research project, a new low-cost

approach is explored to fabricate scintillator crystals using

AAO rather than Si, which enables studies with a wide

range of scintillator size to study the size effect on their

performance for high-sensitivity X-ray detection.





X-Ray X-Ray

Visible light Visible light

Photodetector Photodetector

Scintillator

Reflective layersLight-guided channels

Spatial resolution Spatial resolution

Inte

nsity

Inte

nsity

(a) (b)

Fig. 1. (a) Planar scintillating foil; (b) columnar scintillator with light guides. Note the differences in spatial resolution and signal to noise ratio levels

between them. The weak X-ray radiation is more detectable using the scintillator in (b).

3. RESEARCH ON SCINTILLATORS ATNANOSCALE

When the dimension of material component is reduced

down to nanoscale (<100 nm), enhanced properties

may result, and therefore nanomaterials have increas-

ingly received attention in the past decade. In the field

of scintillation materials, however, there are much fewer

efforts on research at nanoscale as compared with other

types of materials, mainly due to their sample prepara-

tion challenges. Scintillator nanoparticles have been stud-

ied with several different compositions.13�14�43–51 Earlier

work demonstrated that cerium doped Y2SiO5 nanoparti-

cles, in the size range of 25–100 nm, exhibited enhanced

light output, reduced cost and greater size scalability.43�44

The light output of the scintillator nanoparticles was

increased by a factor of three as compared with the bulk

sample.44 Klassen et al. found that nanocrystalline scin-

tillators could improve their property parameters signifi-

cantly, which were promising for applications in a new

generation devices for radiation detections.45 When the

size of BaF2 nanoparticles is less than 80 nm, these par-

ticles exhibited a sharp decrease of self-trapped exciton

luminescence intensity.50 A recent research indicated that

when CsI(Na) particle diameter decreased to nanoscale,

X-ray excited luminescence decay time speeded up sig-

nificantly from ∼600 ns to ∼10 ns.13 It was confirmed

that the diameter reduction led to surface effect, which

accelerated the excitons’ quenching. Such surface induced

speeding detection provides a promising prospect for fast

X- or �-ray detections.

There were no publications about scintillator nanowires

in the open literature until recently Ohashi et al. reported

680-nm-diameter GdAlO3:Ce3+ scintillator fibers sur-

rounded with �-Al2O3, fabricated from directionally solid-

ified eutectics.52 The scintillator fibers converted X-rays

to lights, which were confined and transported along the

fiber directions. Taheri et al. theoretically simulated X-ray

detections by ZnO nanowires in AAO53 and polycarbon-

ate membrane,54 respectively, using Geant4 Monte Carlo

code. According to their simulation results on 200-nm-diameter ZnO nanowires in AAO, it was predicted thatthe AAO template has a special impact as a light guideto conduct the optical photons induced by X-ray towardthe detector thickness and to decrease the light scatter-ing in the detector volume, resulting in improved spa-tial resolution.53 Chen et al. experimentally prepared CsInanowires in AAO.55 Very recently, Taheri et al. reportedan X-ray imager based on ZnO nano-scintillator wires inpolycarbonate membrane with high spatial resolution ofless than 6.8 �m.56

As pointed out by Dujardin et al., the research onnanoscintillators is an open field for studies of excita-tion mechanisms, exploration of new scintillator materials,and applications of active compounds.46 The research onnano-scaling effect on the X-ray luminescence needs largeexploration.

4. THERMODYNAMIC CALCULATION OFCs–I–O SYSTEM

The CsI compound has a sublimation point, melting point,and boiling point at 600 �C, 627 �C, and 1277 �C, respec-tively, under 1 atm. Based on the physical properties ofCsI, no vapors appear below 600 �C under 1 atm. How-ever, when the CsI is heated in a pressure-controlled cham-ber, the vapor can be observed. In our experiment, a largequantity of vapor was clearly observed when the chamberpressure was higher than 1 atm. Some vapor could alsobe seen when the chamber pressure was lower than 1 atm.Therefore, it is necessary to understand the CsI phasesand related vapors present below 600 �C, and evaluatetheir relationship between Cs–I–O vapor and temperaturethrough thermodynamic calculations.According to thermodynamic data from references,57�58

there are 6 different condensed phases and 7 gaseousphases in the Cs–I–O system. The 13 phases are Cs�s, l�,Cs2(g), I�s, l�, I�g� I2(g), O2(g), CsI�s, l�, CsI(g), CsO(g) CsO2�s, l�,Cs2O�s, l�, Cs2O�g�, and Cs2O3�s�. Here, the subscripts g,l, and s represent gas, liquid, and solid, respectively.





Table I. Basic thermodynamic reaction equations and Gibbs free energy

of formation in Cs–I–O system in the temperature range from 25 to

800 �C.

Eqn. reactions �G (kJ/mol)

Cs�s�+ I(s) → CsI(s) �G1 =−3783+009T

Cs(s)+ I(s) → CsI(g) �G2 =−1892−004T

Cs(s)+1

2O2(g) → CsO(g) �G3 = 575−005T

Cs(s)+O2(g) → CsO2(s) �G4 =−2836+014T

2Cs(s)+1

2O2(g) → Cs2O(s) �G5 =−3497+014T

2Cs(s)+ 1

2O2(g) → Cs2O(g) �G6 =−1542−003T

2Cs(s)+3

2O2(g) → Cs2O3(s) �G7 =−5192+025T

I(s) → I�g� �G8 = 762−005T

2I�s� → I2�g� �G9 = 599−014T

Cs�s� → Cs�g� �G10 = 709−007T

2Cs�s� → Cs2�g� �G11 = 941−008T

To evaluate the thermodynamics of the Cs–I–O system,the Gibbs free energy of formation (�Gf) should be cal-culated. The Gibbs free energy of the formation of a

compound is the change in Gibbs free energy from itsconstituent elements. Because �Gf is only a function oftemperature T , the �Gf can be expressed as �Gf = A+BT , where A and B are two numerical numbers. Based

on the available thermodynamic data,57�58 the 13 phasesin Cs–I–O system can be presented as 11 basic thermo-dynamic reaction equations by the Gibbs free energy of

formation, as listed in Table I. The table shows reac-tion compounds of CsI�s�, CsI�g�, CsO�g�, CsO2�s�, Cs2O�s�,Cs2O�g�, Cs2O3�s�, and gases of I�g�, I2�g�, Cs�g�, and Cs2�g�are based on the Cs�s�, O2�g�, and I�s� reactions.

Table II lists five thermodynamic reaction equations,equilibrium constant (K), and Gibbs free energy (�G) ofcesium oxides in the temperature range from 25 to 800 �C.As shown in that table, Cs can be oxidized to Cs2O�s�,Cs2O3�s�, CsO2�s�, CsO�g�, and Cs2O�g�, with Cs:O molar

Table II. Five thermodynamic reaction equations, equilibrium constant (K), and Gibbs free energy (�G) of cesium oxides in the temperature range

from 25 to 800 �C.

Eqn. reactions logK �G (kJ/mol)

2Cs�s�+1

2O2(g) → Cs2O�s� logK5 =−73+ 18266

T�G5 =−3497+014T

2Cs2O�s�+ 1

2O2(g) → Cs2O3�s� logK12 =−56+ 8848

T�G12 =−1695+011T

Cs2O3�s� + 1

2O2�g� → 2CsO2�s� logK13 =−19+ 2506

T�G13 =−48+003T

Cs�s�+1

2O2�g� → CsO�g� logK3 = 28− 3004

T�G3 = 575−005T

2Cs�s�+1

2O2�g� → Cs2O�g� logK6 = 14+ 8052

T�G6 =−1542−003T

ratios of 2, 0.667, 0.5, 1 and 2, respectively. Based on theresults in Table II, the oxygen partial pressures of cesiumoxides can be obtained. For example, the equilibrium par-

tial vapor pressure of oxygen in Cs2O�s� can be calculatedfrom the Gibbs free energy as follows:

2Cs�s�+1

2O2�g� → Cs2O�s� (1)

�G=−3497+014T �kJ/mol� (2)

The molar free energy change �G in this reaction is given

by �G=−RT lnK. Here, K can be expressed in Eq. (3),where aCs2O

, aCs, and PO2represent the activities of Cs2O�s�

and Cs�s�, and partial pressure of O2�g�, respectively. Sincethe activity of a solid is a unity, the relationship between Kand PO2

can be represented in Eq. (4), where K and R are

two constants, and �G is only a function of temperature T .

K = aCs2O

�aCs�2 · �PO2

�05(3)

logK =−1

2logPO2�g�

(4)

Figure 2 shows the thermodynamic curves of cesiumoxides. In Figure 2(a), the thermodynamic stabilities of

oxide compounds in a Cs–I–O system are evaluated byconsidering their Gibbs free energy. In ascending orderof Gibbs free energy in the temperature range of 25 to

800 �C, the compounds are in the order of Cs2O�s� →Cs2O�g� → Cs2O3�s� → CsO2�g� → CsO�g�. Thus, Cs2O�s�

is the most stable phase, and CsO�g� is the least stable

phase. Because Cs2O�s�, Cs2O�g�, Cs2O3�s�, and CsO2�g�

present negative Gibbs free energy values, Cs�s� can be oxi-

dized to Cs2O�s�, Cs2O�g�, Cs2O3�s�, CsO2�g� below 600 �C.In Figure 2(b), Cs can be oxidized to Cs2O�s�, Cs2O3�s�,CsO2�s� with increases in oxygen pressure; for example, Cs

can be oxidized to CsO2�s� under 1 atm (0.2 atm of PO2)

at 600 �C. As shown in Figure 2(c), the partial pressureof Cs2O�g� increases with increases in the temperature and

oxygen pressure, while in Figure 2(d), the partial pressureof CsO�g� decreases with the temperature, but increases

with the oxygen pressure.





(a) (b)

(c) (d)

Fig. 2. Thermodynamic curves of cesium oxides. (a) CsO2�s� has the lowest Gibbs free energy of formation value; (b) Cs can oxidize to Cs2O�s�,

Cs2O3�s�, CsO2�s� with increases in oxygen pressure; (c) the partial pressure of Cs2O�g� increases as temperature and oxygen pressure increase; (d) the

partial pressure of CsO�g� decreases with temperature but increases with oxygen pressure.

Table III outlines the reaction equations, equilibrium

constant, and the formation of Gibbs free energy of I�s�,

CsO�g�, CsO2�g�, Cs2O�s�, Cs2O�g�, and Cs2O3�g� in the tem-

perature range of 25 to 800 �C. Figure 3 further depicts

the formation of Gibbs free energy curves of CsO�g�,

CsO2�g�, I�s�, and I2�g� under 1 atm in the temperature

Table III. Thermodynamic of I�s�, CsO�g�, CsO2�g�, Cs2O�s�, Cs2O�g�, and Cs2O3�g� formation equations in the temperature range from 25 to 800 �C.

Eqn. reactions logK logK �G (kJ/mol)

CsI�s� +1

2O2�g� → CsO�g� + I�s� logK14 = logPCsO�g�

− 1

2logPO2�g�

logK14 = 78− 22764

T�G14 =−4358−014T

CsI�g�+1

2O2�g� → CsO�g�+ I�s� logK15 = logPCsO(g)

− 1

2logPO2�g�

− 1

2logPCsI�g�

logK15 = 05− 12886

T�G15 = 2467−001T

CsI�s� +O2�g� → CsO2�s� + I�s� logK16 =− logPO2�g�logK16 =−24− 4950

T�G16 = 947+005T

CsI�g�+O2�g� → CsO2�s�+ I�s� logK17 =− logPO2�g�− logPCsI�g�

logK17 =−120+ 4928

T�G17 =−944+018T

2CsI�s� +1

2O2�g� → Cs2O�s�+ I�s� logK18 =− 1

2logPO2�g�

logK18 = 27− 21254

T�G18 = 4069−004T

2CsI�g� +1

2O2�g� → Cs2O�s� + I�s� logK19 =− 1

2logPO2�g�

−2 logPCsI�g�logK19 =−119− 1498

T�G19 = 285+022T

2CsI�s� +1

2O2�g� → Cs2O�g� + I�s� logK20 = logPCs2O�g�

− 1

2logPO2�g�

logK20 = 114− 31468

T�G20 = 6024−021T

2CsI�g� +1

2O2�g� → Cs2O�g� + I�s� logK21 = logPCs2O�g�

− 1

2logPO2�g�

−2 logPCsI�g�logK21 =−32− 11712

T�G21 = 2242−005T

2CsI�s� +3

2O2�g� → Cs2O3�s�+ I�s� logK22 =− 3

2logPO2�g�

logK22 =−29− 12406

T�G22 = 2374+007T

2CsI�g� +3

2O2�g� → Cs2O3�s� + I�s� logK23 =− 3

2logPO2�g�

−2 logPCsI�g�logK23 =−175− 7350

T�G23 =−1408+033T

range of 25 to 800 �C. From Figure 3(a), it is seen that

CsO�g�+I�s� would not spontaneously form from CsI�s� +0.5O2�g� and CsI�s�+ 0.5O2�g�; from Figure 3(b), CsO�g�+I2�g� would not spontaneously form from CsI�s�+ 0.5O2�g�

and CsI�s�+ 0.5O2�g�; and from Figure 3(c), CsO2�g�+ I�s�would not spontaneously form from CsI�s� + O2�g�, but





(a) (b)

(c) (d)

Fig. 3. Gibbs free energy formation curves of CsO�g�, CsO2�g�, I�s�, and I2�g�. (a) CsO�g� + I�s� formation from CsI�s� and CsI�g�; (b) CsO�g� + I2�g�formation from CsI�s� and CsI�g�; (c) CsO2�g� + I�s� formation from CsI�s� and CsI�g�; (b) CsO2�g�+ I2�g� formation from CsI�s� and CsI�g�.

CsO2�g� + I�s� can form from CsI�g� +O2�g�. For example,

when the temperature is higher than 600 �C, the solidCsI�s� sublimates to gas CsI�g�, and when the tempera-

ture cools to 277 �C, the gas CsI�g� reacts with O2�g� toform CsO2�g� + I�s�. In Figure 3(d), similar to the reac-

tion in Figure 3(c), CsO2�g�+I2�g� would not spontaneouslyform from CsI�s�+O2�g�, but CsO2�g�+ I2�g� can form from

Table IV. Thermodynamic of I2�g�, CsO�g�, CsO2�g�, Cs2O�s�, Cs2O�g�, and Cs2O3�g� formation equations in the temperature range from 25 to 800 �C.

Eqn. reactions logK logK �G (kJ/mol)]

CsI�s�+1

2O2�g� → CsO�g�+ I2�g� logK11 = logPCsO(g)

+ logPI2(g)− 1

2logPO2(g)

logK24 = 114− 24328

T�G24 =−405−084T

CsI�g� +1

2O2�g� → CsO�g� + I2�g� logK17 = logPCsO�g�

+ logPI2�g�− 1

2logPO2�g�

− 1

2logPCsI�g�

logK25 = 41− 14450

T�G25 = 2767−07T

CsI�s�+O2�g� → CsO2�s� + I2�g� logK18 = logPI2�g�− logPO2�g�

logK26 = 12− 6514

T�G26 = 1247−06T

CsI�g� +O2�g� → CsO2�s� + I2�g� logK19 = logPI2�g�− logPO2(g)

− logPCsI�g�logK27 =−85+ 3364

T�G27 =−644−05T

2CsI�s� +1

2O2�g� → Cs2O�s� + I2�g� logK20 =+ logPI2�g�

− 1

2logPO2�g�

logK28 = 63− 22818

T�G28 = 3769−07T

2CsI�g�+ 1

2O2�g� → Cs2O�s�+ I2�g� logK21 = logPI2�g�

− 1

2logPO2�g�

−2 logPCsI�g�logK29 =−84− 3062

T�G29 = 585−05T

2CsI�s� + 1

2O2�g� → Cs2O�g�+ I2�g� logK22 = logPI2�g�

+ logPCs2O�g�− 1

2logPO2�g�

logK30 = 149− 33032

T�G30 = 5724−09T

2CsI�g�+1

2O2�g� → Cs2O�g�+ I2�g� logK23 = logPI2�g�

+ logPCs2O�g�

− 1

2logPO2�g�

−2 logPCsI�g�logK31 = 035− 13276

T�G31 = 2542−07T

2CsI�s� +3

2O2�g� → Cs2O3�s� + I2�g� logK24 = logPI2�g�

− 3

2logPO2�g�

logK32 = 065− 13970

T�G32 = 2674−07T

2CsI�g�+3

2O2�g� → Cs2O3�s� + I2�g� logK25 = logPI2�g�

− 3

2logPO2�g�

−2 logPCsI�g�logK33 =−1395− 8914

T�G33 =−1108−04T

CsI�g�+O2�g�. For example, when the temperature is higher

than 600 �C, the solid CsI�s� sublimates to CsI�g�, and whenthe temperature cools to 327 �C, CsI�g� reacts with O2�g� to

form CsO2�g�+ I2�g�.

Table IV and Figure 4 outline the reaction equa-tions, equilibrium constants, and formation Gibbs free

energy of I2�g�, CsO�g�, CsO2�g�, Cs2O�s�, Cs2O�g�, and





(a) (b)

(c) (d)

Fig. 4. Gibbs free energy formation curves of Cs2O�s�, Cs2O�g�, I�s�, and I2�g�. (a) Cs2O�s� + I�s� formation from CsI�s� and CsI�g�; (b) Cs2O�g� + I�s�formation from CsI�s� and CsI�g�; (c) Cs2O�s� + I2�g� formation from CsI�s� and CsI�g�; (d) Cs2O�g�+ I2�g� formation from CsI�s� and CsI�g�.

Cs2O3�g� in the temperature range of 25 to 800 �C. In

Figure 4(a), Cs2O�s� + I�s� mixture forms from CsI�s� and

CsI�g�; Figure 4(b), Cs2O�g�+I�s� forms from CsI�s� andCsI�g�; Figure 4(c), Cs2O�s� + I2�g� forms from CsI�s�and CsI�g�; and Figure 4(d), Cs2O�g� + I2�g� forms fromCsI�s� and CsI�g�. All curves show that no spontaneous for-

mation of Cs2O�g�+I�s� and Cs2O�g� + I2�g� from CsI�s� +0.5O2�g� and CsI�g�+0.5O2�g� reactions.

Because CsI has a sublimation point of 600 �C at

1 atm, the CsI presents as a gaseous state above 600 �C.Figure 5 presents that the Gibbs free energy curves of

CsI�s� and CsI�g� decompose to Cs�s�, Cs�g�, I�s�, and I2�g�.In Figure 5(a), the solid state CsI�s� decomposes to Cs�s�+I�s� and Cs�s�+0.5 I2�g�; and in Figure 5(b), the gaseous

state of CsI�g� decomposes to solid Cs�s�+0.5 I2�g� andCs�g�+0.5 I2�g�. All curves show positive Gibbs free energy

(a) (b)

Fig. 5. Gibbs free energy thermodynamic curves of CsI decomposing to Cs and I. (a) Solid state of CsI�s� decomposes to Cs�s� + I�s� and Cs�s� + 0.5

I2�g�; (b) gaseous state of CsI�g� decomposes to solid Cs�s� +0.5 I2�g� and Cs�g�+0.5 I2�g�.

values, indicating that solid CsI�s� and gaseous CsI�g� are

stable under 1 atm in the temperature range of 25 to

800 �C. Table V lists the thermodynamic equations ofCsI decomposing to Cs and I in the temperature range

of 25 to 800 �C. The decomposition temperatures ofsolid CsI�s� to Cs�s�+I�s� and Cs�s�+I2�g� are 3930 �C and

2278 �C, respectively. The decomposition temperatures of

solid CsI�g� to Cs�s�+ I2�g� and Cs�g�+I2�g� are lower, at1919 �C and 1433 �C, respectively. Therefore, the solid

CsI�s� can transform to the gas CsI�g� at temperatures

higher than 600 �C, and CsI�g� decomposes to Cs�g�+I2�g�at temperatures higher than 1433 �C. The above results

show that perfectly solid CsI�s� and gaseous CsI�g� shouldnot spontaneously decompose to Cs�s�, I�s�, and I2�g� below

600 �C at 1 atm. The solid CsI�s� may form gaseous

CsI�g� at temperatures higher than 600 �C, and CsI�g�





Table V. Thermodynamic equations for CsI to decompose to Cs and I

in the temperature range from 25 to 800 �C.

Eqn. reaction �G (kJ/mol) Equ. point ��G= 0�

CsI�s� → Cs�s� + I�s� �GA = 3783−009T T = 3,930 �C

CsI�s� → Cs�s� +1

2I2�g� �GB = 4083−016T T = 2,278

CsI�g� → Cs�s� + 1

2I2�g� �GC = 2192−01T T = 1,919 �C

CsI�g� → Cs�g�+1

2I2�g� �GD = 2901−017T T = 1,433 �C

will decompose to Cs�g�+I2�g� at temperatures higher than

1433 �C.Figure 6 shows the thermodynamic curves of I�g�,

I2�g�, Cs�g�, Cs2�g�, and CsI�g� formation under 1 atm. In

Figure 6(a), the Gibbs free energy curve implies that solid

I�s� can transform to the gases I2�g� and I�g� at 113�C and

1524 �C. In Figure 6(b), the Gibbs free energy curve indi-

cates that solid Cs�s� can transform to the gases Cs2�g� and

Cs�g� at 1176�C and 740 �C. As shown in Figure 6(c), the

sublimation points of CsI are 600 �C and 700 �C at 1 atm

and 1.6 atm; and in Figure 6(d), the sublimation points of

I are 600 �C, 627 �C, and 700 �C at 179 atm, 213 atm, and

323 atm, respectively. In the Clausius-Clapeyron equation,

P , �H , T , R, and C are pressure (atm), molar latent heat

of phase change (J/mole ·K), temperature (K), gas constant

(8.314 J/mole ·K), and integration constant, respectively.

According to the available thermodynamic data,57�58 the

�H of CsI is 3993.9 J/mole ·K at 1 atm and 600 �C, and

(a) (b)

(c) (d)

903 ºC1251 ºC

Fig. 6. Thermodynamic curves of I�g�, I2�g�, Cs�g�, Cs2�g�, and CsI�g� formation. (a) Gibbs free energy curve of I�g� and I2�g� formation; (b) Gibbs free

energy curve of Cs�g� and Cs2�g� formation; (c) CsI sublimation points are 600 �C and 700 �C at 1 atm and 1.6 atm; (d) I sublimation points are 600 �C,627 �C, and 700 �C at 179 atm, 213 atm, and 323 atm, respectively.

thus C can be calculated as 4.57. Therefore, the relation-ship between CsI sublimation point and pressure can beexpressed as:

lnP = −39939

T+457 (5)

It is similarly possible to calculate the relationshipbetween I sublimation point and pressure. The �H of I is41,570.3 (J/mole ·K) at 1 atm and 184 �C, and thus C canbe calculated as 10.94. Therefore, the relationship betweenI sublimation point and pressure can be express as:

lnP = −415703

T+1094 (6)

The above results show that both CsI�g� and I2�g� vaporsare formed below the CsI�s� melting point (627 �C) at 1 atm;only I2�g� but no CsI�g� vapor is formed below the CsI�s�melting point (627 �C) at 1.15 atm; CsI�g� and I2�g� vaporsdo not form below the CsI melting point (627 �C) above213 atm. The thermodynamic calculations suggest that novapors appear when the temperature is below 600 �C. How-ever, in our experimental procedure, vapor was clearlyobserved from the quartz window of chamber at 400 �C.This phenomenon may be related to defective CsI powders,or due to the reactions of CsI�g�+O2�g� →CsO2�g�+I2�g� andCsI�g�+O2�g� → CsO2�g�+I�s� that may occur.

5. EXPERIMENTAL PROCEDURESThe experimental procedures include:(a) fabrication of an AAO template with a pore size of450∼500 nm;





(b) deposition of CsI nano-particles on the AAO inner

wall;

(c) formation of CsI columns inside the AAO channel by

vacuum, positive pressure, and vacuum mechanical pres-

sure methods; and

(d) microstructure observations of CsI columns using

scanning electron microscopy (SEM).

The following details describe the formation of AAO, CsI

nano-particles, and CsI columnar crystals

(1) Fabrication of sub-micron channels of the AAO tem-

plate. AAO film was made with the following anodization

procedures:

(a) electrolytic polishing of Al substrate

using electrolyte of 15 vol% HClO4 + 15 vol%

CH3(CH2�3OCH2CH2OH + 70 vol% C2H6O under an

applied voltage of 42 V (DC) at 20 �C for 10 min;

(b) 1st anodization using 1 vol% H3PO4 and 200 V at

1 �C for 30 min;

(c) removal of the first anodized film using 1.8 wt%

CrO3+6 vol% H3PO4+92 vol% H2O at 70 �C for 40 min;

(e) second anodization using 1 vol% H3PO4 under

200 V at 1 �C for 10 h;

(f) pore widening using 5 vol% H3PO4 at 25 �C for

4 h; and

(g) removal of Al substrate using 8 wt% CuCl2 +5 vol% HCl+85 vol% H2O, at 25

�C for 3 min. After the

anodization procedure, AAO templates with a pore diam-

eter of ∼450 nm, film thickness of ∼120 �m, and pore

density of 5×109 pore/cm2 were obtained.

(2) Pretreatment of CsI nano-particles. Because of capil-

larity, the CsI solution can easily penetrate the AAO film,

so that CsI nanoparticles can be deposited on the submi-

cron channels. In order to improve the wettability between

CsI melt and AAO channel, the CsI nanoparticles were

first deposited on the AAO inner pore walls by a wet depo-

sition method in a solution with 1 wt% CsI at 25 �C for

3 min, and then the sample was sintered in an atmosphere

furnace (400 �C for 5 min) to increase the adhesion of CsI

particles on AAO walls.

(3) Formation of CsI columnar crystals. To form the CsI

columns, difference approaches were used as follows:

(a) inside the injection chamber, place AAO template

(with CsI nanoparticles adhered) on the bottom and CsI

powders on the top surface of AAO template. After pump-

ing to vacuum and heating at 650 �C for 5 min, the CsI

melt was mechanically pressed into the AAO by a hydrol-

ysis force;

(b) on a slice of quartz sheet place a piece of AAO

template and add CsI powders on the surface of AAO

template, and move the CsI/AAO to a positive heating

chamber and heated as 650 �C for 5 min so that the CsI

melt flew into the AAO template, and the chamber was

filled with Ar to maintain pressure of 1 at, 3 atm and

25 atm, respectively;

(c) place the set of CsI powders/AAO template into theinjection chamber and applied with a positive pressure of1.6 atm with Ar, and after heating at 650 �C for 5 min,

the CsI melt was mechanically pressed into the AAO bya hydrolysis force.

The microstructure and composition of the fabricatedsamples were studied using optical microscopy (Nikon LV

150), X-ray Diffractometer (XRD, Philips X’Pert Pro).),and Scanning Electron Microscopes (SEM, JEOL 7400and 6510) equipped with EDS.

6. ELECTROCHEMICAL BATH ANDMOLD DESIGNS

Since the AAO process is sensitive to the operation condi-

tions, defects may appear in AAO if unsuitable conditionsare used. The conditions include electrolyte temperature,applied voltage, electrolyte composition, electrolyte stir-

ring, and current density distribution. In an anodizationprocess, the working electrode presents an exothermicreaction (�H < 0). For example, when Al is anodized in

H3PO4 aqueous solution, the reaction equations can beexpressed as:

H3PO4 → 3H++PO−34 � �H0

f =−12774 kJ (7)

Al→ Al+3� �H0f =−5314 kJ (8)

H2O→ H++OH−� �H0f =−2301 kJ (9)

All the reactions in Eqs. (7)–(9) are exothermic because

the standard enthalpies of �H0f are negative. The heat of

the exothermic reaction in Eq. (7) can be removed beforeanodization when the electrolyte is cooled in a cooling

bath. However, the exothermic reactions of Eqs. (8) and(9) occur during anodization, causing local heat. The localheat should be removed quickly by a cycling or agitat-

ing electrolyte; otherwise, local cracking, pits, defects, orburning may appear on the AAO surface. The exothermiceffects are especially obvious when a higher voltage isapplied, as in the case of 200 V applied to produce submi-

cron pores in AAO. Therefore, it is necessary to carefullydesign the electrochemical mold and bath for the produc-tion of good quality AAO.59

Figure 7 shows the electrochemical mold for AAO fab-rication. Figure 7(a) presents the components, includingengineering plastics, O-ring, copper rod, copper plate,

and silicone sheet. Figure 7(b) is an exploded diagram;Figure 7(c), a combination diagram; and Figure 7(d), aphoto of an actual electrochemical mold. The advantages

of this mold include convenient loading and unloading ofthe sample, easy control of sample reaction area, uniformcurrent density on the sample surface, and prevention ofelectrolyte leakage. The applied voltage determines the

AAO pore diameter and barrier layer thickness. Two expe-rience formulas can estimate the pore size and the poredistance of AAO. The pore size with voltage is: C =mV,





(a)

(b) (c) (d)

Fig. 7. Electrochemical mold for AAO fabrication. (a) Components including engineering plastics, O-ring, copper rod, copper plate, and silicon chip;

(b) exploded diagram; (c) combination diagram; (d) actual electrochemical mold.

where C is cell size (nm), V is anodizing voltage (V),

and m is a constant (2–2.5). The pore distance with volt-

age is V = �2R− 10�/2, where 2R is spacing distance

(5∼1000 nm).60

Because of the exothermal reaction during anodization,

it is difficult to make a thick AAO film with submicron

pore size. In order to make a thick AAO film with big pore

diameter, the electrode mold must be redesigned. Figure 8

illustrates a novel cooling-function of electrochemical

mold. As shown in Figure 8(a), the design of cooling

in/out electrode can remove reaction heat quickly. In the

exploded diagram in Figure 8(b), the structure includ-

ing cooling in/out electrode made from brass, O-ring as

water sealing, and insulated mold made from engineering

(a) (b) (c) (d)Cooling in

Cooling out

Cooling in/out

Electrode

Insulated mold

Active surfaceCooling in/out electrode

Water sealing

Fig. 8. Novel cooling-function of electrochemical mold. (a) Cooling in/out electrode; (b) exploded diagram; (c) combination diagram; (d) actual

cooling-function electrochemical mold.

plastic. Figure 8(c) is the appearance of combined parts,

including insulated mold, electrode connecting to appliedvoltage, anodization active area, and cooing in/out tubes.

Figure 8(d) is a photo of an actual cooling-function elec-

trochemical mold. The materials and process for the moldare convenience at a low cost, and the mold can be made

in a mass production easily.Figure 9 schematically shows the electrochemical bath

system. In Figure 9(a), the system structure comprises

an air pump, water pump, reaction bath, heat exchangepipe, heat exchange bath, and cooler. In order to quickly

remove the heat of the exothermic reaction and keep the

electrolyte in an isothermal state during anodization; thechemical reaction bath, cooling bath, and heat change bath





Fig. 9. Electrochemical bath system. (a) System structure comprising

of an air pump, water pump, reaction bath, heat exchange pipe, heat

exchange bath, and cooler; (b) the electrochemical bath; (c) trombone

heat exchanger.71

are connected together. Figure 9(b) is a chemical reaction

bath, which consists of electrolyte input and output ports,an air input port, and pumping vacuum ports. The elec-trolyte inputs from the cooling bath are diverted to four

inlets. This allows the chemical reaction bath to maintaina more constant temperature and electrolyte agitation than

a design that has only one electrolyte inlet. Figure 9(c)is a heat exchange tube structure, which consists of pipes,elbows, and connecting tubes made of silicone gel. This

flexible structure can be extended to form various shapesto fit the heat change bath.

7. ANODIC ALUMINUM OXIDE (AAO)TEMPLATE

Anodic alumina has been called by various names,

such as anodic aluminum oxide (AAO),61–65 anodic alu-mina nanoholds (AAN),66 anodic alumina membrane

(AAM),67�68 or porous anodic alumina (PAA).69 High-quality AAO templates with high pore density, uniformpore diameter, and ordered nanochannel arrangement can

be fabricated by the electrochemical process. The meltingpoint of AAO is near 1,000 �C,70 and the AAO template is

stable around 800 �C,66 which is a lower temperature thanthat of bulk alumina (2017 �C for Al2O3��) and higherthan that of the CsI melt (627 �C).57 It is believed that

the melting point of AAO is lower than that of pure alu-mina because of inclusions in the porous structure AAO.For example, the elements S, C, and P are doped into

AAO during anodization using H2SO4, HOOCCOOH, andH3PO4 as the electrolytes.

For any electrolyte, factors of a higher anodizing volt-age, a lower electrolyte temperature, or a lower acidconcentration favor film growth. Oppositely, factors of a

lower applied voltage, a higher anodizing temperature, or

a higher acid concentration cause film dissolution. When

the rate of film growth has fallen to the same rate of dis-

solution of the film in the electrolyte, the thickness of the

film remains the same, i.e., the film does not grow fur-

ther. Of many electrolytes and applied voltages used to

produce AAO, the most common ones are 10 vol% sul-

furic acid (18 V), 3 wt% oxalic acid (40 V), and 1 vol%

phosphoric acid (200 V), which are used to fabricate AAO

with pore sizes of 10–30 nm, 40–90 nm, and 180–500 nm,

respectively.71

Figure 10 shows photographs of step by step of AAO

sample fabrication. The starting material is a piece of pure

aluminum foil, with 0.3 mm thickness, 4 inch diameter

and 99.999% purity (Fig. 10(a)). This Al foil was mechan-

ically grounded and polished to remove its surface oxida-

tion layer (Fig. 10(b)). After electrolytic polishing using

the conditions given in the reference,71 its surface turns to

be smooth and shiny (Fig. 10(c)). This piece of sample

was anodization for the first time (Fig. 10(d)) and then the

ordered pattern was removed (Fig. 10(e)) using the condi-

tions given in the reference.71 The sample was anodized for

the second time (Fig. 10(f)). After cutting off the remain-

ing area removed (Figs. 10(g) and (h)), the Al substrate

was dissolved using chemical solutions described in the

previous article.71

Figure 11 demonstrates some SEM images of the AAO

templates. In Figure 11(a) along the AAO top view, the

pore open size is about 480 nm in diameter. The inserted

EDS spectrum reveals elements of Al, O, and P. After

the barrier layer is removed, the pores on the bottom

of the AAO are similar to those on the top, as shown

in Figure 11(b). From the lateral views in Figure 11(c),

the AAO channels are straight with a high aspect ratio

(250 �m thickness and 0.48 �m pore diameter). As

shown in Figure 11(d), the maximum pore diameter can

potentially be expanded to 520 nm because the AAO

has a pore diameter of 450 nm and pore wall thickness

of 150 nm.

When phosphoric acid is dissolved in water, more

H2PO−4 and fewer HPO2−

4 and PO3−4 ions are present in the

aqueous solution. The ion concentrations can be calculated

based on the dissociation constants Ka1, Ka2, Ka3, where

Ka1 = 71×10−3, Ka2 = 63×10−8, and Ka3 = 10×10−18

are the first, second, and third dissociation constants of

the reaction equations in Eqs. (10)–(12), respectively.60 In

our anodization process, the electrolyte content is 1 vol%

(0.16 M) H3PO4 with 1.55 pH value at 0 �C. According to

Eqs. (10)–(12), the ion concentrations of [H+], [H2PO−4 ],

[HPO2−4 ], and [PO3−

4 ] are 28× 10−2 M, 41× 10−2 M,

92×10−8 M, and 33×10−24 M, respectively.72

H3PO4⇔H++H2PO−4 Ka1

= [H3O+��H2PO

−4 �

�H3PO4�(10)





(a) (b) (c)

(d) (e) (f)

(g) (h) (i)

3 cm

Fig. 10. Photographs of step by step of AAO sample fabrication. (a) Starting material of a piece of aluminum foil with 0.3 mm thickness, 4 inch

diameter and 99.999% purity; (b) the substrate through mechanical polishing; (c) after electropolishing; (d) after 1st anodization; (e) after removing

the ordered pattern of the first anodization layer; (f) after 2nd anodization; (g) and (h) after cutting off the remaining material; (i) AAO film without

Al substrate.

(d)(c)

(a) (b)

Fig. 11. SEM images of AAO microstructure. (a) AAO top view with 480 nm pore diameter, and EDS spectrum inserted showing Al, O, and P

elements; (b) AAO bottom view showing the same pore size on the top view; (c) AAO with 250 �m film thickness; (d) AAO with 450 nm pore

diameter and 150 nm pore wall thickness.





H2PO−4 +H2O⇔H3O

++HPO2−4 Ka2

= �H3O+��HPO2−

4 �

�H2PO−4 �(11)

HPO2−4 +H2O⇔H3O

++PO3−4 Ka3

= �H3O+��PO3−

4 �

�HPO2−4 �

(12)

It is easy to prepare a small sample of thin AAO filmwith nano-sized pores. However, a large-area sample ofAAO film with a submicron pore size and a long thicknessis difficult to produce with a regular anodization process.In order to increase the CsI light emission efficiency andavoid cross-talk between CsI columns, we made a 4-inchdiameter size of AAO film with 480 nm pore size and250 �m film thickness by using an anodization processand the electrochemical system of our design.In the literature, the large AAO channels can be fab-

ricated using photolithography method, which has beenestablished for a wide range of applications.73–75 The sam-ple fabrication process of CsI columnar crystals in AAOis schematically shown in Figure 12, which includes thefollowing steps:(a) The starting material is Al with high purity(>99.99%);(b) Anodization of Al. Highly ordered AAO nanoporeswith diameters from 10–450 nm can be prepared usinganodization of pure aluminum metal in an electrochemicalbath.71�76�77 The length of the AAO channels is controlledby the anodization time.

(b)

Al Al

(a) (c)

(d) (e) (f)

(g) (h) (i)

Nail Polish

Al substrate removed

Photoresist Large AAO ChannelsCoating

CsI(Tl) Melt

CsI(Tl) CrystalsCsI(Tl) Scintillator Crystals

Nanochannels

CsI(Tl) Melt

Fig. 12. Sample fabrication process. (a) Staring material of pure Al; (b) anodization to obtain AAO nanochannels; (c) removing remaining Al by

mechanical method followed with chemical dissolution, and the nanochannels are sealed with nail polish; (d) flipping the sample and depositing with

photoresist; (e) performing photolithography to obtain larger AAO channels; (f) coating AAO channels; (g) mechanically injecting CsI(Tl) melt into

the AAO channels at a high temperature; (h) CsI(Tl) melt solidifies; (i) removing extra samples on the surface.

(c) Mechanically remove remaining Al if it is toothick, followed with chemical dissolution using NaOH

solution.71 If smaller AAO channels are needed, move to

step (f); otherwise for larger AAO channels, seal the openside of the nanochannels with nail polish;

(d) Flip over the sample, deposit with photoresist;(e) Perform photolithography to obtain larger AAO

channels;

(f) Coat AAO channels with metal oxide particles toimprove the light reflection. This can be done using chem-

ical deposition, such as Cu,78 Ni,79 or Co80 compounds, as

well as atomic layer deposition (ALD);81–87

(g) Mechanically inject CsI(Tl) melt into the AAO

channels;(h) When the sample cools down, CsI(Tl) melt solidifies;

(i) Mechanically remove extra samples on the surface,

and CsI(Tl) scintillator crystals within AAO channels areyielded.

The channel walls should be coated with reflective

layers to polish the wall surface.2�37 Badel et al. sug-gested coating Si wall with ruthenium (Ru) by ALD.37

SiO2 can also be selected for coating by ALD, since it

has a low refractive index (1.46 for the blue light), sothat light in CsI(Na) (refractive index 1.84) at the inci-

dence angle (angle between light and the interface normal)larger than the critical angle of sin−1�146/�184�� =525� will be totally reflected. However, for the samples

prepared by anodization with smaller size (≤500 nm),the ALD coating is not needed. The refractive index of





(a) (b)

Ni3P coating

Ni3P coating

Fig. 13. AAO channels coated with Ni3P layers along top (a) and bottom (b) views.

Al2O3 wall is 1.65, so the light will be totally reflected

when the angle is greater than the critical angle ofsin−1�165/�184��= 637�.

Figures 13(a) and (b) show top and bottom views of thecoating layers within AAO, respectively. The AAO chan-

nels are coated with Ni3P layers using electroless chemicaldeposition, including

(1) sensitization: 0.3 wt% SnCl2+ 25 vol% HCl+H2O,at 25 �C for 3 min;

(2) activation: 0.1 wt% PdCl2+ 10 vol% HCl+H2O, at25 �C for 3 min; and

(3) electroless deposition: 2 wt% NiSO4 + 2 wt%NaH2PO2+2 ppm Pb(NO3�2+2 wt% Na citrate or citric

acid, at 70 �C and pH = 5 for 3∼30 min. It is seen thatthe bottom size is closed, and therefore, the bottom side

should be mechanically removed before melt injection.

8. COLUMNAR CsI FABRICATION BYNEGATIVE PRESSURE INJECTION

Columnar CsI crystals with a smooth and flat surface

favor the conversion of X-ray to visible light. However,the regular CsI is soft and extremely hygroscopic; it is

very difficult to polish to obtain a smooth and optical flatplane. In order to obtain a high-quality CsI scintillator

for X-ray application, we used an ordering channel as thetemplate and formed sub-micron CsI columns in the tem-

plate. Figure 14 schematically illustrates the fabrication of

(a)

AAO/CsI

Barrier layer

GlassCsI Liquid

(b)

(c)(d)

(e)

AAO/CsI AAO/CsIAAO AAO/CsI

Barrier layerAl

Barrier layerAl

Barrier layerAl

Barrier layerAl

CsI Raw Material

Glass

Fig. 14. Schematic step-by-step fabrication procedure. (a) AAO; (b) CsI

nanoparticles solidified on AAO walls, (c) before mechanical injection;

(d) after mechanical injection; and (e) final AAO/CsI. Reprinted with

permission from [55], C. C. Chen, et al., Anodic-aluminum-oxide tem-

plate assisted of cesium iodide (CsI) scintillator nanowires. Mater. Lett.112, 190 (2013). © 2013, Elsevier.

CsI nanowires by the mechanical injection method usinghighly ordered AAO (Fig. 14(a)) as template. As men-tioned previously, it is necessary to deposit CsI nanopar-ticles on AAO walls first using dip-dry method. Aqueoussolution containing 10 wt% CsI is dropped on the AAOtemplate, which is then placed on a hot plate to dry at100 �C. Afterwards, CsI nanoparticles are deposited onthe AAO inner walls (Fig. 14(b)). Since the melting pointof CsI is relatively low (627 �C), the mechanical injec-tion method can be adopted to fabricate CsI nanowiresin AAO.71�88–95 The schematic apparatus can be found inprevious publications.71�88 Inside of the vacuum chamber(∼10−2 torr) of the mechanical injection device, AAO tem-plate with CsI nanoparticles is placed on the bottom side,and raw material of CsI particles are on its top (Fig. 14(c)).When the device is heated up above the melting tempera-ture of CsI, a hydraulic force (∼10 kgf/cm2) is applied toinject the molten CsI into the nanopores of the AAO tem-plate (Fig. 14(d)). After the injection process, the chamberis kept in vacuum to cool down slowly. The remainingAl substrate can be removed mechanically, followed withdissolution in an etching solution of 10 wt% CuCl2 and8 vol% HCl until Al is dissolved, and the solidified CsIlayers covering on the top surface of AAO should also beremoved mechanically (Fig. 14(e)).The ionic bonds in CsI can be dissolved in a polar-

ized H2O forming aqueous solution. When the solutiondeposited on AAO dries on, CsI nanoparticles form onAAO. Figure 15 shows SEM images of CsI nanoparticlessolidified on top surface of AAO (Figs. 15(a) and (b)),and bottom of AAO walls (Figs. 15(c) and (d)), formed byimmersing AAO into 1 wt% CsI solution and dried on ahot plat at 100 �C.An XRD pattern of the CsI/AAO sample is shown in

Figure 16(a). Peaks are from both CsI and Al substrate,as well as impurity byproducts of Al2O3 and AlPO4. TheEDS spectrum in Figure 16(b) confirms the elements inCsI, as well as Al, O, C and P elements. When Al isanodized in H3PO4 containing aqueous solution, the hydro-gen (H+) ions are absorbed on the cathode forming hydro-gen gas (H2); while on the other electrode, the OH−,H2PO

−4 , HPO

2−4 , and PO3−

4 ions are absorbed on the anode





(a)

(d)

(b)

(c)

Fig. 15. SEM images of CsI particles (arrowheads indicated) solidified on AAO walls. (a) CsI nanoparticles on the AAO walls; (b) partial CsI on AAO

pores; (c) partial and discontinue CsI wires inside AAO top; and (d) CsI particles at the bottom of AAO. Reprinted with permission from [55], C. C.

Chen, et al., Anodic-aluminum-oxide template assisted of cesium iodide (CsI) scintillator nanowires. Mater. Lett. 112, 190 (2013). © 2013, Elsevier.

and reacts with aluminum (Al3+) ion forming phosphor

doped aluminum hydroxide Al(OH)3) or boehmite (Al2O3 ·H2O) films. The film can further transform to alumina

film after heat treatment. The element C is from the AAO

template.

Dendritic microstructure always presents during the

conventional solidification, as shown in Figure 17. When

1 wt% CsI solution is dried on the surface of a glass slide,

dendritic structures are formed, as shown in Figure 17(a)

and its enlargement in Figure 17(b). However, when CsI

is solidified from the melt (630 �C) to 25 �C on a glass

slide in an air furnace, both dendrites and bubble pits are

(d)(d)

C

O

Al

P

ICs

CsCs

Cs

(b)

0 1 2 3 4 5 6

Energy (keV)

0

10

20

30

40

20 30 40 50 60 70 80

Inte

nsity

(×1

000)

Two-theta (degree)

Al: JCPDF 03-0932AlPO4: JCPDF 78-2444

♦ CsI: JCPDF 06-0311

(110)

(321)(211)

(200)

(220)(300)

(111

)(1

13)

(054)

(a)

Fig. 16. XRD (a) and EDS spectrum (b) of CsI nanoparticles solidified on AAO walls. Reprinted with permission from [55], C. C. Chen, et al.,

Anodic-aluminum-oxide template assisted of cesium iodide (CsI) scintillator nanowires. Mater. Lett. 112, 190 (2013). © 2013, Elsevier.

formed, as shown in Figure 17(c) and a picture show-

ing the pits only is presented in Figure 17(d). Whether

CsI solidifies from the aqueous solution or melt on the

glass slides, dendrites can grow in the three-dimensional

(3D) free space to form 3D patterns, without any exter-

nal restrictions to their growth. The bubble-shape pits are

associated with the gaseous vapors evaporated from the

melt during the solidification process.

Using the AAO pre-deposited with CsI nanoparticles

as template, CsI melt was mechanically injected into the

AAO nanochannels under a vacuum negative pressure.

Both larger (Figs. 18(a)–(c)) and smaller (Fig. 18(d))





(a) (b)

(c) (d)

50 μm

50 μm

100 μm

50 μm

Fig. 17. Optical images of CsI with dendritic structure solidified from 1 wt% CsI solution (a), (b) and melt (c), (d). Reprinted with permission

from [55], C. C. Chen, et al., Anodic-aluminum-oxide template assisted of cesium iodide (CsI) scintillator nanowires. Mater. Lett. 112, 190 (2013).© 2013, Elsevier.

(a) (b)

(c) (d)

Fig. 18. SEM images of CsI nanowires. (a) Partially retained CsI on AAO surface; (b) CsI nanowires filled in AAO channels; (c) lateral view of

CsI/AAO; (d) cross-sectional view of a fracture surface showing single CsI nanowires. Reprinted with permission from [55], C. C. Chen, et al.,

Anodic-aluminum-oxide template assisted of cesium iodide (CsI) scintillator nanowires. Mater. Lett. 112, 190 (2013). © 2013, Elsevier.





nanowires were fabricated, with diameters of 450 nm and100 nm, respectively. In Figure 18(a), partial CsI layerretains on top of AAO, and in Figure 18(b), AAO is almostcompletely filled with CsI nanowires. From the lateralview in Figure 18(c), the larger CsI nanowires almost filledthe entire channels over the length. From the fracture ofCsI-filled AAO film in Figure 18(d), highly ordered CsInanowires are obtained. It should be mentioned that if theCsI nanoparticles were not pre-deposited on AAO, verylimited AAO nanochannels could be filled with CsI bymechanical injection. The pre-deposited CsI reduced thecontact angle between the CsI melt and AAO. It is interest-ing that when CsI melt is confined in AAO nanochannels,it grows as stable single wires without any dendrites, con-sistent with previous theoretical predictions.96

9. COLUMNAR CsI CRYSTAL FABRICATIONBY POSITIVE PRESSURE

Besides to the mechanical inject under negative pressure,CsI melt was also mechanically injected into AAP poresunder positive pressure filled with Ar.97 Figure 19 showsthe design of the chambers to prepare the CsI columns.Figure 19(a) is an exploded diagram of the positive pres-sure chamber. The chamber structure includes a pressuregauge, pneumatic valve, gasket between flanges, stainlessscrews between space chamber and flange, rubber seal-ing between flanges and the quartz window, and a ther-mal couple. Figure 19(b) is a photo of an actual positivepressure chamber. The chamber structure included a pres-sure gauge, a pneumatic valve, gaskets between flanges,stainless screws between the space chamber and flanges,a cooling cycler, and an injection column. Figure 19(c)shows an exploded diagram of the injection chamber, and

(a) (b)

(c) (d)

Fig. 19. Positive pressure chambers for CsI fabrication. (a) Exploded

diagram; (b) a photo of the actual positive pressure chamber; (c) exploded

diagram of the injection chamber; (d) a photo of the actual injection

chamber.

Figure 19(d) is a photo of an actual injection chamber.According to the thermodynamic calculations, the subli-mation point of CsI is at 600 �C at 1 atm, which is lowerthan its melting point of 627 �C. The CsI sublimation pointcan be raised to 700 �C at 1.6 atm, a temperature that ishigher than the melting point of 627 �C. Therefore, wemade a positive chamber to fabricate the CsI columns, andsuch a non-vacuum process could reduce the amount ofCsI raw material consumed during the injection process.Figure 20 shows SEM images of CsI columns in the

AAO channel with certain content of moisture which wasintroduced during the SEM sample preparation in mois-ture atmosphere for overnight. As shown in Figure 20(a),since the CsI sample was prepared in moisture ambient forSEM observation, the CsI absorbed moisture and expandedin volume, and some of the CsI came out of the AAOsurface, forming hemisphere shapes. From the side viewsimages, CsI volume expanded partially (Fig. 20(b)) orlargely (Fig. 20(c)). Figure 20(d) is a side view image ata low magnification showing the CsI volume expansion. Itis known that the CsI is a hygroscopic material that dis-solves by absorbing moisture vapor in air. CsI has a highsolubility in water, and the solubility increases as tempera-ture rises. For example, the solubility of CsI in H2O is 44,58, 76, 96, 124, 150, 190, and 205 wt% at 10, 20, 30, 40,60, 80, and 90 �C, respectively.98 Therefore, to producea high-quality CsI material, contact with water and highhumidity should be avoided, especially for submicron CsIcolumns.In order to obtain a good SEM image of the CsI col-

umn, the SEM sample must be prepared in a dry box, suchas a vacuum chamber or a glove box. Figure 21 showsSEM images of CsI columns inside the AAO template,prepared in a glove box. In Figure 21(a), the AAO film wasbroken so the CsI tips protruded from the AAO templatesurface. Figure 21(b) is a side view image showing CsIcolumns in the AAO template. It is found that there is noevident bonding or attraction between AAO and CsI. Thislack of bonding between them favors the construction ofCsI columns throughout the AAO channels. Figure 21(c)is a side view image showing CsI columns protruded fromthe AAO template, and Figure 21(d) is a side view imageshowing a single CsI column pulled out of the AAO tem-plate during the sample preparation, while others remainin the AAO channels.According to the thermodynamic calculation results,

solid iodine and cesium iodide have sublimation points of113 �C and 600 �C to form I2�g� and CsI�g� gases, respec-tively. These points are lower than the CsI�l� melting pointof 627 �C at 1 atm. At a positive pressure of 25 atm ina heating chamber, the I2�g� and CsI�g� formation temper-atures raise to 377 �C and 1886 �C, respectively. Whenthe chamber pressure is increased to 213 atm, the I2�g�and CsI�g� formation temperatures increase to 628 �C andseveral thousand �C. Without a special device and pro-tections, it is difficult and dangerous to make a 213 atm





(a) (b)

(c) (d)

1 μm

5 μm30 μm

1 μm

Fig. 20. SEM images of CsI columns when the SEM sample was prepared in a moist condition. (a) Top view showing CsI partial volume expansion;

(b) side view showing CsI partial volume expansion; (c) side view showing CsI large volume expansion; (d) low-magnification image of a side view

showing CsI volume expansion.

positive chamber, but it was possible to make a 25 atm

positive chamber in our experiment. In order to under-

stand the effects of I2�g� and CsI�g� vapors on CsI col-

umn morphology, we filled CsI melt in an AAO template

(a) (b)

(c) (d)

5 μm

20 μm 2 μm

5 μm

Fig. 21. SEM images of CsI columns without bonding with AAO template. (a) Top view of CsI tips on the AAO template; (b) side view of CsI

columns in the AAO template; Reprinted with permission from [97], C. Y. Chen, et al., Using positive pressure to produce a sub-micron single-crystal

column of cesium iodide (CsI) for scintillator formation. Mater. Lett. 148, 138 (2015). © 2015, Elsevier. (c) side view of CsI columns coming out of

AAO template surface; (d) side view of single CsI columns pulled out of the AAO template.

under 1 atm, 3 atm, 25 atm, and mechanical injection

conditions.Figure 22 shows SEM images of CsI columns in

the AAO templates produced by various fabrication





(a) (b)

(c) (d)

5 μm

5 μm 5 μm

5 μm

Fig. 22. SEM images of CsI columns in AAO template produced by various fabrication conditions. (a) CsI with partial filling and discontinuous

columns prepared under 1 atm; Reprinted with permission from [97], C. Y. Chen, et al., Using positive pressure to produce a sub-micron single-crystal

column of cesium iodide (CsI) for scintillator formation. Mater. Lett. 148, 138 (2015). © 2015, Elsevier. (b) CsI with discontinuous columns prepared

under 3 atm; (c) CsI with a few portion of discontinuous columns prepared under 25 atm; (d) CsI with continuous columns prepared by mechanical

injection under 1.6 atm. Reprinted with permission from [97], C. Y. Chen, et al., Using positive pressure to produce a sub-micron single-crystal column

of cesium iodide (CsI) for scintillator formation. Mater. Lett. 148, 138 (2015). © 2015, Elsevier.

conditions. In Figure 22(a) under 1 atm process, the dis-

continuous CsI columns partially fill the AAO template.

Because the I2�g� and CsI�g� vapors presented before the CsI

melting point, the vapors inside the AAO channels occu-

pied the space, causing discontinuous and a partially-filled

morphology. When the pressure was increased to 3 atm, as

shown in Figure 22(b), the filling was improved as com-

pared with 1 atm. Such filling was further improved under

25 atm, as shown in Figure 22(c), and most of the CsI

columns presented a continuous morphology. However,

with the aid of mechanical injection of the CsI melt under

1.6 atm at 650 �C, almost all of the CsI columns filled the

AAO channels, exhibiting a continuous morphology. The

sample of CsI columns inside the smooth AAO walls can

be used for the high-energy radiation detection with high

light emission for pixelated scintillator applications.

10. CONCLUSIONS

In this paper, we reported thermodynamic characteriza-

tions of CsI-based materials and fabrication of submicron-

sized CsI columns by injecting CsI melts into the

AAO channels. We integrated the thermodynamic calcula-

tions, positive chamber design, mold design, electrochem-

ical bath design, anodization process, and solidification

techniques to fabricate the submicron CsI columns. Ele-

ment I and compound CsI sublimate at 113 �C and 600 �Cto form I2�g� and CsI�g� vapors at 1 atm pressure, respec-

tively. However, at 25 atm pressure the I and CsI subli-

mation points can be increased to 377 �C and 1886 �C,respectively. If the sublimation point is higher, the amount

of CsI raw material needed for the injection could be

reduced. Because the I2�g� and CsI�g� vapors presented

below the CsI melting point, partial and discontinuous

CsI columns formed in the AAO template under 1 atm.

However, the filling quality of the CsI columns could be

improved to full and continuous columns under a positive

pressure of 25 atm, or with the aid of mechanical injection.

Vacuum mechanical injection also produced good filling

of CsI columns, while more CsI raw materials were con-

sumed during this process.

The ionic compound CsI columns are difficult to obtain

by electrochemical ionic deposition method. Although var-

ious methods were used to fabricate different nanowires

in the past,99–105 the mechanical injection method seems

to be the most suitable one to fabricate the CsI nanowires

which it does not require any chemical precursors. Their

size can be controlled by the AAO pores, and chemical

doping level (such as Tl or Na), by the composition of

starting raw materials. When the CsI melt is confined to





an AAO channel with a high aspect ratio, it grows as a sta-ble single column with smooth surfaces, free of dendritesand grain boundaries. This liquid CsI melt-filling methodwith anodic template assistance allows fabrication of CsIcolumns inside the smooth AAO walls that can be used forthe high-energy radiation detection with high light emis-sion for pixelated scintillator applications.

Acknowledgments: The work at NUT and NCYUTwas financially supported by the Chung-Shan Instituteof Science and Technology (CSIST) under Contract No.CSIST-442-V103 and the National Science Council, Tai-wan, under Contract No. 103-2221-E-239-004. The workat FSU was supported by NSF HRD-1436120, and DoDW911NF-14-1-0060 and W911NF-09-1-0011. N. Noeland N. Juliely were FSU-RISE scholars.

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