Surface Science 609 (2013) 183–189

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Surface Science

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Thiol dosing of ZnO single crystals and nanorods: Surface chemistryand photoluminescence

Jagdeep Singh a, Jisun Im a, Evan J. Watters a, James E. Whitten a,⁎, Jason W. Soares b, Diane M. Steeves b,⁎⁎a Department of Chemistry and Centers for Advanced Materials and High-Rate Nanomanufacturing, The University of Massachusetts Lowell, Lowell, MA 01854, United Statesb U.S. Army Natick Soldier Research, Development & Engineering Center, Natick, MA 01760, United States

⁎ Corresponding author. Tel.: +1 978 934 3666; fax:⁎⁎ Corresponding author. Tel.: +1 508 233 4320; fax:

E-mail addresses: James_Whitten@uml.edu (J.E. Whdiane.m.steeves.civ@mail.mil (D.M. Steeves).

0039-6028/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.susc.2012.12.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 2 October 2012Accepted 7 December 2012Available online 19 December 2012

Keywords:AdsorptionPhotoemissionPhotoluminescenceRaman spectroscopyThiolWork functionZinc oxide

Adsorption of thiols on ZnO(0001) and ZnO nanorods has been investigated using X-ray and ultraviolet photoelec-tron spectroscopies (XPS and UPS). Ultrahigh vacuum (UHV) dosing of sputter-cleaned ZnO(0001) withmethanethiol (MT), 1-dodecanethiol (DDT), and 3-mercaptopropyltrimethoxysilane (MPTMS) leads to S2p peakswith a binding energy of 163.3 eV. Similar results for MPTMS are obtained for sputter-cleaned ZnO(0001) that ispre-dosedwithwater to formhydroxyl groups. In all cases, the absence of a free thiol S2p peak at 164.2 eV indicatesthat bonding to the surface occurs via the thiol end of the molecule. A DDT-dosed ZnO(0001) sample stored for10 days in UHV and heated to temperatures as high as 150 °C exhibits minimal changes in its S/Zn atomic ratio,confirming chemisorption and the presence of a strong bond to the surface. UPS shows that MT adsorption onsputtered ZnO(0001) leads to a 0.7 eV increase in work function and perturbation of the MT molecular orbitals,again consistent with chemisorption. Dry ZnO nanorods have been exposed to MT while monitoring theirphotoluminescence. XPS and Raman spectroscopy confirm thiol adsorption. Relative to dry ZnO, adsorption causesa decrease in intensity of the visible emission peak, but the UV peak remains unchanged. These results indicate thatZn\S bond formation quenches radiative decay to the valence band fromdefect states, possibly bymethanethiolateadsorption filling oxygen vacancies.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Zinc oxide is one of the most-studied metal oxides. There is arenewed interest in the surface chemistry and physics of ZnO singlecrystals and nanocrystals due to optoelectronic device applicationsthat include light-emitting diodes, lasers, photodiodes, solar cells,field-effect transistors, gas sensors, biosensors, and piezoelectrics[1]. ZnO is wurtzite in crystal structure, and (0001) single crystalsmay be commercially obtained with either zinc surface termination,designated ZnO(0001), or oxygen surface termination, designatedZnO(000 �1). Unique optical properties arise from its wide bandgapof 3.2 eV at 300 K [2] and efficient radiative recombination [1]. Inaddition to an ultraviolet emission peak due to an excitonic recombi-nation, nanocrystalline ZnO also exhibits a yellow-to-green visibleemission peak whose origin is still somewhat controversial [1,3].ZnO surfaces are reactive to a variety of gases, including carbonmonoxide, carbon dioxide, water, pyridine and methanethiol [4–9].Our interest is in using adsorption to tailor the optical properties

+1 978 934 3013.+1 508 233 5521.itten),

rights reserved.

of metal oxides and possibly to fabricate chemical sensors based onchanges in photoluminescence.

Halevi and Vohs [6] studied methanethiol adsorption at roomtemperature on ZnO(0001) and showed that dissociative adsorptionoccurs, resulting in chemisorption of methylthiolate. Thermal desorp-tion spectroscopy indicated strong bonding, with desorption occur-ring above 400 °C. In contrast, for ZnO(000�1), associative adsorptionresults. Consistent with these findings is work by Sadik et al. [7]in which O- and Zn-terminated ZnO substrates were immersedin 1-dodecanethiol/isopropanol solutions; thermal stability wasobserved up to 350 and 400 °C, respectively. Pesika et al. [8] havestudied the quenching of ZnO nanoparticle growth by 1-octanethioland compared it to adsorption on as-received single crystals follow-ing immersion in 1-octanethiol solutions. The authors measuredappreciable adsorption but concluded that it was reversible. Quartzcrystal microbalance studies by Joo et al. [9] of adsorption of1-hexanethiol on a sputter-deposited ZnO substrate indicate adsorp-tion with a packing density of ca. 1.5×1015 molecules/cm2. It shouldbe noted that this is comparable to thiol adsorption on gold surfaces[10], confirming that adsorption on ZnO does not simply occur atdefects. Theoretical work also confirms enthalpically driven adsorp-tion. Jena and coworkers [11] studied methanethiol adsorption onZnO nanostructures. Their work indicates that on-top Zn sites favorchemisorption, and their calculations predict that the nanoparticlesbecome ferromagnetic due to thiol adsorption.

184 J. Singh et al. / Surface Science 609 (2013) 183–189

In the present study, we have investigated adsorption, inultrahigh vacuum, of methanethiol (MT), 1-dodecanethiol (DDT) and3-mercaptopropyltrimethoxysilane (MPTMS) on sputter-cleanedZnO(0001). MPTMS is of interest since it could potentially bond toZnO(0001) via either the silane or thiol end of the molecule. In allcases, chemisorption is demonstrated using X-ray photoelectron spec-troscopy (XPS), with the formation of Zn\S bonds. We also presentthe first ultraviolet photoelectron spectroscopy (UPS) investigation ofthiol adsorption on zinc oxide. It is found that the MT frontier orbitalsare strongly perturbed by adsorption on ZnO(0001), with the workfunction of the surface increasing by 0.7 eV. Exposure of ZnO nanorodpowder to MT also leads to thiolate bonding. XPS and Raman spec-troscopies confirm adsorption, and in-situ photoluminescence mea-surements show that the intensity of the visible emission peak isdecreased by methanethiolate adsorption. It is postulated that thisis due to passivation of surface-related defect states, such as oxygenvacancies.

2. Materials and methods

2.1. ZnO(0001) single crystal studies

ZnO(0001) single crystals from MTI Corporation were attached tocopper XPS sample stubs using vacuum compatible, high temperaturesilver adhesive (Aremco, Inc.) and loaded into the introduction cham-ber of a VG ESCALAB MKII photoelectron spectrometer. After exten-sive heating and outgassing at ca. 150 °C, the ZnO single crystal washeated to 600 °C and sputtered using 1000–5000 eV argon ions. Thesputter cleaning was carried out in the preparation chamber of theinstrument, which had a vacuum lower than 1×10−9 Torr. Aftercleaning, the sample was rapidly moved to the analysis chamber,where the pressure was ca. 5×10−10 Torr. XPS was used to confirmsample cleanliness, and it was dosed with gases (after cooling towithin ca. 15° of room temperature) by backfilling the chamberwith either MT, MPTMS, and DDT or water vapor through variableleak valves. Exposures of 50–90 L of thiol were achieved by dosingthe ZnO(0001) at a pressure of 1×10−7 Torr. As will be discussed,in some cases the surface was dosed with 100 L of water prior toMPTMS adsorption by backfilling at a pressure of 5×10−7 Torr for200 s. No corrections were made for ionization gauge cross sectionsin measuring these pressures. Several freeze-pump-thaw cycles hadbeen used to purify MPTMS, DDT and water, stored in glass reservoirsbehind their respective variable leak valves, and residual gas analysesconfirmed their purities. Sample heating and temperature monitoringwere accomplished with a commercial sample heater (VGmodel 240)into which a carousel holding multiple samples could be inserted. Athermocouple was attached to a sample stub on the carousel to mon-itor the temperature.

2.2. ZnO nanorod studies

ZnO nanorods (Nanocerox, Inc.) were used for these studies. Trans-mission electron microscopy showed them typically to be 50–100 nmwide and 100–700 nm long. They were placed in the middle of a4 mm inner dia., 6 mm outer dia. Suprasil tube (HS-04601 fromFriedrich & Dimmock, Inc.) that was installed in the sample compart-ment of a Horiba Jobin Yvon Fluorolog II fluorometer operating infront-face reflection mode, with an excitation wavelength of 325 nm.The ZnO nanorods were held in place with a small plug of glass wool.A stainless steel valve arrangementwas used such that either dry nitro-gen or MT gas (Sigma-Aldrich) could be flowed through the Suprasiltube while fluorescence spectra were collected. The gas was flowedthrough the tube at a flow rate of 67 cm3/min. The following procedurewas used: 1) dry nitrogenwas flowed through the ZnO nanorod samplefor 60 min while monitoring photoluminescence; 2) the nitrogen gasflow was terminated, and MT was immediately flowed for 5 min;

3) the MT flow was terminated, and nitrogen was flowed for 60 minto purge the sample of physisorbed MT, during which time photo-luminescence was monitored. The ZnO sample was then placed in acapped bottle for further analysis. For XPS, sample preparationconsisted of attaching a piece of vacuum compatible, double-sidedadhesive tape onto a sample stub and pressing it into the powdersuch that the entire surface of the tape was covered. A control sample,consisting of as-received ZnO nanorod powder was similarly prepared.Both sampleswere loaded into the XPS, stored overnight at a vacuumofca. 1×10−9 Torr, and analyzed the following day. For Raman spectros-copy, the powder was analyzed with a Bruker Senterra Raman micro-scope system using an excitation wavelength of 532 nm.

2.3. Photoelectron spectroscopy

XPS and UPS were performed using MgKα X-rays (hν=1253.6 eV) and He I UV radiation (hν=21.22 eV), respectively, inthe photoelectron spectrometer. The kinetic energy of the ejectedphotoelectrons was measured with a concentric hemispherical ana-lyzer operating in fixed analyzer transmission mode; pass energiesof 20 and 2 eV, respectively, were used for XPS and UPS. Unless oth-erwise stated, XPS and UPS were performed at a take-off angle(TOA) of 90°, defined as the angle between the sample plane andthe normal to the entrance of the focusing lens of the electron energyanalyzer. For XPS, the samples were electrically grounded, while forUPS they were held at a bias potential (hereafter referred to as“bias”) of ca. −6.40 V (accurately measured) such that the kineticenergy (KE) of the lowest energy photoelectrons could be acceleratedenough to be measured. For one experiment, a UPS spectrum of con-densed MT was obtained by cooling a copper sample to ca. 135 K andleaking in approximately 50 L of the gas. This permitted the spectrumof MT chemisorbed on ZnO(0001) to be compared to multilayer,physisorbed MT.

For UPS, the binding energy (BE) was referenced to the Fermi levelof the spectrometer (which equals that of the metal sample stub)using the following equation:

BE ¼ hν−KE− Biasð Þ: ð1Þ

The UPS spectrum could be re-referenced to the vacuum level byrigidly shifting it such that its high BE cutoff was equal to the photonenergy (21.22 eV). In that case, the x-axis was referred to as the “ion-ization energy”. The work function for a semiconductor, such as ZnO,is the energy difference between the Fermi level (EF) and vacuumlevel. In contrast to a metal, the edge of the valence band does notcoincide with EF.

XPS spectra of nanorod samples were corrected for surface charg-ing by aligning Zn3p peaks with the accepted binding energies andshifting other peaks by an equivalent amount.

3. Results and discussion

3.1. ZnO(0001) single crystal studies

Prior to adsorption, as-received ZnO(0001) single crystals weresputter-cleaned using argon ions. Sputtering removed most of theadventitious carbon from the surface, as demonstrated in Fig. 1a.Fig. 1b displays the corresponding O1s spectra. The O1s peak at531.0 eV and the shoulder at 533.0 eV, for the as-received ZnO(0001)sample, are assigned to bulk oxygen and adsorbed surface hydroxyl spe-cies, respectively. The peak binding energy of the O1s peak aftersputtering is 530.4 eV, in agreement with the expected value for theO2− core level of ZnO [4]. Sputtering removes essentially all of theadsorbed hydroxyl groups on the ZnO(0001) surface. CleanedZnO(0001) was subsequently dosed with 50 L of DDT or MT by leakingthe respective vapors into the UHV chamber. Fig. 1 includes C1s, O1s,

185J. Singh et al. / Surface Science 609 (2013) 183–189

S2p and Zn3p XPS plots followingDDT andMTadsorption. The C1s peakposition at a binding energy of ca. 285 eV is due to aliphatic carbonatoms in the methyl and methylene groups. In both cases, an S2p bind-ing energy of 163.3 eV is observed, consistent with previous studies inwhich ZnO(0001) was dosed in UHV with methanethiol [12].

Adsorption of 3-mercaptopropyltrimethoxysilane (MPTMS) wasalso studied with the goal of understanding whether bonding toZnO(0001) occurs via thiol or silane groups. MPTMS adsorbed onthe sputter-cleaned ZnO(0001) surface leads to two unresolved C1speaks centered at ca. 285.5 eV and 287.0 eV. The peak at 285.5 eV isdue to C\C, C\S and C\Si species, while that at 287.0 eV arisesfrom carbon atoms bonded to oxygen atoms in intact methoxygroups, as shown in Fig. 1a. Adsorption of MPTMS on ZnO(0001)causes a high binding energy shoulder in the O1s spectrum, due tothe methoxy groups, as shown in Fig. 1b. In the case of MT and DDT,the absence of the higher binding energy C1s and O1s peaks, as is

Fig. 1. MgKα XPS of the a) C1s, b) O1s, c) S2p and d) Zn3p regions of sputter-cleaned1-dodecanethiol (DDT), and 3-mercaptopropyltrimethoxysilane (MPTMS). The O1s and ZnTOA was 90°, and the binding energy is referenced to the Fermi level.

observed for MPTMS, confirms those conclusions. Fig. 1c and dshow the S2p and Zn3p regions, respectively. The observed S2p bind-ing energy is 163.3 eV, with a FWHM of 2.1 eV. The value correspond-ing to “free thiol” (i.e., condensed, unreacted thiol in which SH groupsare intact) is 164.2 eV [13]. This is a significant difference and sug-gests that MPTMS bonds to the surface exclusively via the thiolgroups. It should be noted that for the data in ref. [13], no charge cor-rection to the XPS binding energy scale was necessary because thecondensed layers were thin enough to avoid surface charge buildupduring analysis. Therefore, direct comparison of the binding energiesin that work and in the present study is valid.

Experiments were also performed in which sputter-cleanedZnO(0001) was dosed with 100 L water prior to MPTMS adsorption.Water is known to dissociate at room temperature on ZnO(0001) toform a hydroxyl-covered surface [4]. The question addressed is:Does the presence of OH groups on the surface affect the manner in

ZnO(0001) and a similarly prepared surface after dosing with methanethiol (MT),3p spectra of as-received ZnO(0001) prior to sputter cleaning are also included. The

186 J. Singh et al. / Surface Science 609 (2013) 183–189

which MPTMS bonds to the surface? Fig. 2a shows data for a very sur-face sensitive 10° TOA experiment. Identical S2p peak binding ener-gies of 163.0 eV and FWHM of ca. 2.5 eV for samples with andwithout water dosing indicate that there is no difference in the orien-tation of the adsorbed MPTMS, with MPTMS bonding to the surfacewith the sulfur attached to Zn. Fig. 2b shows a comparison of theSi2p region for 10° and 90° TOAs for MPTMS adsorbed on bothsputter-cleaned and water-dosed ZnO(0001). The Si2p peak is moreprominent for the more surface-sensitive, smaller TOA. The back-ground, which arises from the ZnO substrate, overwhelms the signalat 90° because of the larger detection depth. These data confirmthat MPTMS adsorbs with the sulfur toward the surface, even in thecase of hydroxyl-covered ZnO(0001).

Petoral et al. [14] functionalized SiC, GaN and ZnO by exposingthem to MPTMS vapor. Bonding exclusively via the silane end of themolecule was observed by XPS for SiC and GaN. For ZnO epilayersdeposited on a sapphire substrate, the authors concluded that 30–50%

Fig. 2.MgKα XPS of the a) S2p and b) Si2p regions of MPTMS-dosed ZnO(0001). In onecase the sputter-cleaned sample was exposed to water vapor prior to dosing withMPTMS, as described in the text. In the case of the S2p region, a 10° TOA was used.In the case of the Si2p experiments, data are included for 90° and 10° TOAs, as indicat-ed in the figure. The binding energy is referenced to the Fermi level.

of the molecules adsorbed via the silane end of the molecule while theremainder attached via thiolate formation. They postulated that silanebonding occurred to hydroxylated sites, and thiol bonding occurred toZn sites on the surface. Our results are not in accord with these conclu-sions, but the differences may be due to differences in the preparationconditions (dosing in a UHV chamber in our case, in which very littlewater is present during dosing, versus placing the samples in a chamberat 6 mbar and 423 K sealedwithMPTMS, in the case of ref. [14]). Tracesof water during MPTMS adsorption in ref. [14] could have led toself-polymerization, partial multilayer formation and the presence offree thiol groups.

In order to assess the thermal stability of thiols adsorbed onZnO(0001), a DDT-dosed ZnO(0001) sample was stored for 10 days inUHV and then heated in vacuum, with XPS recorded after heating atvarious temperatures. Initially, the S-to-Zn atomic ratio was 0.063.Note that this value is much less than unity due to the approximatedetection depth of 50 Å of the XPS experiment, with a monolayer of Sbeing on the surface and Zn being in the bulk and near-surface region.Long-term vacuum storage of the sample removed physisorbed thiolmolecules, with the S/Zn ratio decreasing to 0.053 after 10 days.Heating of this sample to 150 °C, however, resulted in negligiblechange, confirming that DDT is chemisorbed on the surface.

Previous studies by Halevi and Vohs [6,15] showed that the reactionof CH3SH with ZnO surfaces is structure-dependent, with dissociativeand associative adsorption occurring at 300 K on ZnO(0001) andZnO(000�1), respectively. In the case of ZnO(0001), the authors hypoth-esized that dissociative adsorption occurs on exposed cation-anion sitepairs on terrace and step edges, with methanethiolate adsorptionoccurring on exposed Zn sites and proton adsorption occurring on Osites. Thermal desorption spectroscopy identified the dominant desorp-tion pathway as consisting of CH3SH desorption (between 325 and575 K) resulting from recombination of methanethiolate species andadsorbed protons. Minor pathways were also identified that consistedof (CH3)2S desorption (peaked at 510 K) with concomitant formationof adsorbed sulfur, and CH2O desorption (peaked at 560 K) via decom-position of adsorbed methoxy (CH3O), arising from exchange of sulfuratoms, in adsorbed methanethiolate, with lattice oxygen. Heating to800–900 K desorbed Zn and SO2, with the latter arising from sulfuradsorbed at lower temperatures. These results are consistent with ourobserved stability of DDT in UHV and after heating to 150 °C.

He I UPS was performed to investigate changes in the electronicstructure of the valence states due to methanethiol adsorption onZnO(0001). Fig. 3a and b show UPS spectra, plotted with respect tothe Fermi level, of sputter-cleaned ZnO(0001) before and after dosingwith 50 L of methanethiol. The large peak in each spectrum at ca.15–16 eV is due to nondescript secondary electrons. The same spectra,plotted with respect to the vacuum level, are shown in Fig. 3c and d.Clean ZnO(0001) exhibits peaks at 9.8, 12.6, and 15.9 eV, with respectto the vacuum level. These are due to non-bonding O 2p orbitals, abonding combination of O 2p and Zn 4s orbitals, and the Zn 3d band, re-spectively [16]. UponMT exposure, peaks at 8.8, 11.7, and 15.8 eV dom-inate the spectrum. A spectrum of condensed, multilayer MT is alsoincluded in Fig. 3c. This exhibits molecular orbital peaks at 8.0, 10.2,11.3, and 13.9 eV. We assign these, respectively, to the 3a″ (ns charac-ter), 10a′ (σCS character), 9a′ (σSH character) and 2a″+8a′ (πCH3 char-acter) orbitals based on theoretical gas-phase calculations [17], with anempirically determined polarization shift of 1.7 eV applied to all orbitalsto bring them into agreement with the gas phase ones. The significantdifferences between the spectrum of MT/ZnO(0001) compared to con-densed MT are consistent with perturbation of adsorbate orbitals bychemisorption.

The threshold of the highest occupied state (HOS) for each spec-trum in Fig. 3c is often referred to as the ionization potential (IP) ofthe surface and corresponds to the valence band edge relative to thevacuum level. Issues related to calculating the work function of zincoxide surfaces have been previously discussed [18,19]. To calculate

187J. Singh et al. / Surface Science 609 (2013) 183–189

the work function, it is necessary to locate EF relative to the valenceband edge. This is conveniently accomplished by locating the HOSthreshold in a spectrum plotted with respect to EF (referred to asVBF). The work function (Φ) is then calculated as:

Φ ¼ IP–VBF: ð2Þ

For sputter-cleaned ZnO(0001) andMT-dosed ZnO(0001), the workfunction values are 3.4 and 4.1 eV, respectively. The value for cleanZnO(0001) is in the (large) range of values measured previously forthis surface [20]. However, as pointed out in ref. [20], the measuredvalue depends not only on the nature of the surface but also on themethod used to measure it. Chemisorption causes a 0.7 eV increase inthe work function of the surface.

Fig. 3. He I UPS spectra of sputter-cleaned ZnO(0001) before and after dosing with methanethe vacuum level. a) Shows the entire spectrum, while b) shows the region near the Fermirespect to the vacuum level, and a spectrum of condensed MT is also included. d) Shows eexamples included of how they were determined.

Only a limited number of studies have been carried out related towork function measurements of adsorption on ZnO(0001). It wasfound that room temperature saturation dosing with ammonia leadsto a work function decrease of ca. 0.2 eV [21], but it is not clearfrom that paper how the molecule bonds to the surface. Rodriguez[22] performed semi-empirical quantum mechanical calculations ofvarious small molecules on Zn13O13 to understand the details oftheir chemisorption on ZnO(0001) surfaces. In the case of CH3O,HCOO and OH, the adsorbed species resemble the correspondinganions (i.e., CH3O\, HCOO\, and OH\), and on-top site adsorptionmay be considered as consisting of two steps: 1) electron transferfrom the substrate to the LUMO of the adsorbate, and 2) subsequentmixing of the occupied orbitals of the adsorbate with the empty onesof the substrate. The examined radicals were found to act as overallnet electron acceptors and to cause an increase in the work function

thiol (MT). a) and b) are referenced to the Fermi level, while c) and d) are referenced tolevel. c) Shows a portion of the same spectra as in a), but the data are replotted withxpansion of the region near the vacuum level. Thresholds are suitably indicated, with

157158159160161162163164165166167168169170

Phot

oele

ctro

n C

ount

s

Binding Energy (eV)

ZnO Nanorods

MT/ZnO Nanorods

S2p

Fig. 4. MgKα XPS of the S2p region of as-received and MT-dosed ZnO nanorods.

188 J. Singh et al. / Surface Science 609 (2013) 183–189

of the surface. It is reasonable to assume that CH3S would behave simi-larly to methoxy (since the dipole moments of gas phase methanol andmethanethiol are somewhat similar), and the observed increase inwork function is consistent with the results of ref. [22].

3.2. Nanorod studies

Fig. 4 displays XPS results for exposure of ZnO nanorods tomethanethiol gas. The broad S2p peak at ca. 163 eV (FWHM=2.8 eV)is consistent with thiolate formation, but the larger FWHM than wasobserved for the single crystal studies suggests less homogeneousadsorption and poorer ordering. Raman spectroscopy of ZnO nanorodsbefore and after MT exposure is shown in Fig. 5. The peak at ca.141 cm−1 is ascribed to a Zn\S stretch, and that at 641 cm−1 arisesfrom a C\S stretch [23]. The peak at 2939 cm−1 originates from CH3

stretches [23], and the peaks at 3594, 3633, and 3675 cm−1 are due

Fig. 5. Raman spectra of as-received ZnO nanorods and a similar sample after exposure toregion from 2800–3800 cm−1. A 532 nm laser was used as the radiation source.

to adsorbed hydroxyl groups [24]. The emergence of these peaks fol-lowing MT adsorption suggests the following adsorption reaction.

O latticeð Þ þ CH3SH gð Þ ¼ CH3S adsð Þ þ OH adsð Þ: ð3Þ

This is consistent with the pathway outlined by Halevi and Vohs[15] for dissociative adsorption of MT on clean ZnO(0001) at 300 Kin which chemisorption occurs via methanethiolate and hydroxylgroup formation. PL spectra of ZnO nanorods under several conditionsare shown in Fig. 6. This figure displays the emission spectrum in airof as-received ZnO nanorods and the same sample while purging withdry nitrogen gas. Purging causes an increase in the intensity of the UVpeak and a decrease in the intensity of the visible one. These changesmost likely result from exclusion of oxygen from the sample and/orremoval of trapped water. To separate these effects, an experimentwas performed (not shown) in which a batch of nanorods was purgedwith dry nitrogen and then exposed to oxygen. This caused the UVemission peak to decrease and the visible one to increase; the effectwas completely reversible. Therefore, the phenomena seen in Fig. 6are not due to exclusion of oxygen from the surface. It is concludedthat removal of physisorbed water (i.e., room temperature drying)leads to enhancement of the UV peak and a decrease in the visibleone. Xie et al. [25] measured PL of nanoscale ZnO dried at varioustemperatures and combined these studies with thermal desorptionspectroscopy measurements. The authors observed that low temper-ature heating (i.e., lower than 200 °C) leads to enhancement of theUV peak and a decrease in the visible one, with thermal desorptionof water between 100 and 600 °C. These results are in accord withours and confirm that physisorbed water enhances visible emissionat the expense of UV excitonic emission.

Fig. 6 also shows the effect of exposing nitrogen-purged ZnOnanorods tomethanethiol gas and then purging to remove physisorbedMT. Chemisorption results in maintenance of the UV peak and adecrease in the visible one. van Dijken [3] has proposed one of themore commonly accepted mechanisms regarding the origin of theZnO visible emission peak, postulating that it arises from nonradiativetransfer of an excited electron from the conduction band to a lowerlying defect-related interband state. Decay of this state to the valenceband gives rise to visible emission, with competition existing betweenradiative UV excitonic and visible emission. Decay from the interband

MT, as discussed in the text. a) Shows the region below 750 cm−1, and b) shows the

350 400 450 500 550 600

Phot

olum

ines

cenc

e In

tens

ity

Wavelength (nm)

N2-dried ZnO Nanorods

N2-dried ZnO Nanorods + MT

As-Received ZnO Nanorods

Fig. 6. Photoluminescence spectra of as-received ZnO nanorods (measured in air), thenanorods after and while flowing dry nitrogen gas over them, and the nanorods afterexposure to MT, but measured while flowing nitrogen gas over them. The excitationwavelength was 325 nm.

189J. Singh et al. / Surface Science 609 (2013) 183–189

state may also occur nonradiatively. The observed decrease in intensityof the visible peak indicates that Zn\S bond formation quenches thedefect state, leading to nonradiative decay. In a previous study [26] byour groups, ZnO nanorods were functionalized with mercaptosilaneligands by stirring colloidal suspensions of the nanorods and ligands;adsorption occurred primarily via Zn\S bond formation. In that case,it was observed that the UV peak increased while the visible peakdecreased due to thiol adsorption. These results are consistent withFig. 6 if the MT/ZnO sample is compared to the as-received ZnO one.

4. Conclusions

Thiols, including mercaptosilanes, strongly adsorb from the gasphase onto sputter-cleaned ZnO(0001) and nitrogen-dried ZnOnanorods. In all cases, adsorption occurs via the formation of Zn\Sbonds, as evinced by XPS. UPS studies on MT adsorption ZnO(0001)indicate strong perturbation of the MT orbitals and an MT-inducedwork function increase of 0.7 eV. In the case of MT adsorption onZnO nanorod powder, Raman spectroscopy confirms MT adsorption,

and photoluminescence measurements carried out under nitrogenshow enhancement of the UV and a decrease of the visible emissionpeaks compared to as-received nanorods measured in air. Theseeffects result from a combination of the exclusion of air and oxygenand apparent quenching of surface states arising from defects, suchas oxygen vacancies.

Acknowledgments

This work was supported by the US Army Natick Soldier Re-search, Development and Engineering Center under contract #W911QY-10-2-0001. This document has been approved for unlimiteddistribution.

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