Spin-Cast Deposition of CdSe-CdS Core-Shell Colloidal Quantum Dots on Doped GaAs Substrates: Structural and Optical Characterization

IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 6, NO. 4, JULY 2007 413

Spin-Cast Deposition of CdSe-CdS Core-ShellColloidal Quantum Dots on Doped GaAs Substrates:

Structural and Optical CharacterizationAdrienne D. Stiff-Roberts, Member, IEEE, Wanming Zhang, Jian Xu, Hongying Peng, and Henry O. Everitt

Abstract—The detailed study of the effects of spin recipe andGaAs substrate doping (i.e., semi-insulating, n-type, or p-type)on the structural and optical properties of spin-cast CdSe-CdScore-shell CQDs provides insight into the surface adsorption andcharge transfer mechanisms that will influence any potentialoptoelectronic device. The hypotheses of this study are: i) it ispossible to establish spin-casting recipes that yield a thin film ofCQDs with large surface density and uniform size, and ii) it ispossible to control the optical response of CQDs by varying theGaAs substrate doping to influence charge transfer processes. Asa result of these measurements, we have been able to demonstratea strong dependence of spin-cast CQD structural properties onthe doping type of the GaAs substrate, as well as evidence frommeasured optical properties to support the idea that hot carriersphotoexcited in the GaAs substrate are transferred either to theCQD surface states through organic surface ligands or directly toconfined states within the CQD.

Index Terms—Charge transfer, colloidal quantum dots, hybridjunctions, photoluminescence.

I. INTRODUCTION

COLLOIDAL quantum dots (CQDs), also referred to assemiconductor nanocrystals or nanoparticles, can be tai-

lored for precise size, shape, and optoelectronic properties dueto the physical confinement of bulk material through carefullycontrolled chemical reactions [1]–[3]. Currently, electroopticand photonic devices comprising CQDs [4]–[15] usuallyfeature polymer/nanocrystal blends, or nanocomposites, to sen-sitize polymers at a specific energy corresponding to the CQDsize. Typically, these nanocomposites are deposited on glasssubstrates in the form of a sol-gel [16], adsorbed layer [14], orLangmuir–Blodgett thin film [17]. It is desirable to develop amethod by which CQDs can be embedded in a semiconductormatrix for optoelectronic devices. Such CQD active regionshave the potential to improve device performance significantly

Manuscript received April 3, 2006; revised March 6, 2007. This work wassupported in part by the National Science Foundation under Grant 0547273.The review of this paper was arranged by Associate Editor A. Bose.

A. D. Stiff-Roberts and W. Zhang are with the Department of Electrical andComputer Engineering, Duke University, Durham, NC 27708 USA (e-mail:[email protected]).

J. Xu is with the Department of Engineering Science and Mechanics, Penn-sylvania State University, State College, PA 16802 USA.

H. Peng was with the Department of Physics, Duke University, Durham, NC27708 USA. She is now with the Ceramic Processing Laboratory, GE GlobalResearch, Niskayuna, NY 12309 USA.

H. O. Everitt was with the Department of Physics, Duke University, Durham,NC 27708 USA. He is now with the U. S. Army Aviation and Missile Research,Development, and Engineering Center, Redstone Arsenal, AL 35898 USA.

Digital Object Identifier 10.1109/TNANO.2007.896845

due to: i) the ability to yield highly uniform ensembles ofnanostructures through size-filtering, and ii) the simplificationof device design models since quantum-sized effects are relatedto the spherical shape of CQDs. In addition, CQDs embeddedin a semiconductor matrix would enable a greater selectionof active region materials for optoelectronic devices since thechemical synthesis of CQDs eliminates strain considerations.The challenge to this approach is in realizing a multilayerheterostructure comprising aqueous CQDs embedded in GaAs,for example. To date, there have been few demonstrations ofCQDs embedded in epitaxial semiconductor materials [18],[19], and it is important to develop a better understanding ofthe structural and optical properties of such material systemsif these heterostructures are to be used in devices. In addition,there have been at least two reports [20], [21] of CQD micro-cavity emitters fabricated using spin-cast deposition, yet nocharacterization was provided of how the spin-casting processaffects CQDs and device performance.

The purpose of this paper is to characterize the structuraland optical properties of spin-cast CdSe-CdS core-shell CQDson GaAs substrates for application to optoelectronic devices.The detailed study of the effects of spin recipe and GaAssubstrate doping on spin-cast CQDs provides insight into thesurface adsorption and charge transfer mechanisms that willinfluence any potential device. The hypotheses of this studyare: i) it is possible to establish spin-casting recipes that yielda thin-film of CQDs with large surface density and uniformsize, and ii) it is possible to control the optical response ofCQDs by varying the GaAs substrate doping to influencecharge transfer processes. Previous work supporting theseideas include the characterization by photoluminescence (PL)spectroscopy and scanning tunneling microscopy of CdSeCQDs deposited on 1,6-hexanedithiol-coated, doped GaAssubstrates by direct adsorption [22]; the characterization byelectrostatic force microscopy of CdSe-CdS CQDs spin-cast onhighly ordered, pyrolitic graphite and doped Si substrates [23];and the characterization by PL spectroscopy and transmissionelectron microscopy (TEM) of InAs CQDs deposited on GaAssubstrates by direct adsorption [18]. In this work, CdSe-CdScore-shell CQDs are spin-cast on semi-insulating, n-type, andp-type GaAs substrates and are subsequently characterizedby atomic force microscopy (AFM), PL spectroscopy, andphotoluminescence excitation (PLE) spectroscopy. As a resultof these measurements, we have been able to demonstrate astrong dependence of spin-cast CQD structural properties onthe doping type of the GaAs substrate, as well as evidence from

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414 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 6, NO. 4, JULY 2007

measured optical properties to support the idea that hot carriersphotoexcited in the GaAs substrate are transferred either to theCQD surface states through organic surface ligands or directlyto confined states within the CQD.

Thus, charge transfer mechanisms must be considered indevice performance, and may in fact enable unique devicedesign when the doping concentration of the semiconductorsubstrate or epitaxial layer is controlled. In particular, animportant challenge to realizing photonic devices featuringCQDs in a semiconductor matrix is controlling doping and/orelectrical charge injection. Forster-like energy transfer in-volving dipole-dipole interactions has been proposed as thelikely mechanism for charge transfer from a semiconductorepitaxial layer (such as a quantum well) to an organic overlayeror CQD overlayer [24], [25]. An important requirement forthis Forster-like energy transfer is the energetic overlap of theepitaxial layer emission spectrum and the overlayer absorptionspectrum. However, in the present study, the epitaxial layeris replaced by a bulk GaAs substrate whose peak emissionenergy is much lower than the CQD peak absorption energy.Forster-like energy transfer cannot occur because the CQDoverlayer is transparent to the GaAs substrate emission. Thus,an important contribution of this letter is the identificationof another charge transfer mechanism: photoexcitation of hotcarriers in the GaAs substrate followed by charge transfer toeither surface or confined states of the CQDs.

II. SPIN-CASTING OF CQDS ON GaAs SUBSTRATES

The methods developed by Peng et al. [26] are used for thechemical synthesis of CdSe-CdS core-shell CQDs character-ized in this study. The as-synthesized CdSe-CdS CQDs aredissolved in chloroform ( 5–10 mg/ml) and have the followingdimensions determined by inorganic reaction times: CdSe corediameter 3.5–4.5 nm and CdS shell thickness 3 mono-layers. In addition, organic surface ligands (tri-octyl phosphineoxide—TOPO) encapsulate the CQDs to prevent agglomera-tion when in solution and to provide electrical isolation of theCQDs. At this point, it is also useful to consider the opticalproperties of the as-synthesized CQDs in solution. Fig. 1 showsthe room-temperature, normalized PL intensity as a functionof detection energy for a range of excitation energies. The PLspectrum peaks at 2.10 eV with a full-width half-maximumlinewidth ranging from 116–125 meV. The inset to Fig. 1shows the PLE spectrum corresponding to the as-synthesizedCdSe-CdSe CQDs. The PLE intensity versus excitation en-ergy is detected at an energy blue-shifted from the peak CQDemission energy, as denoted on the PL spectrum, in order toincrease the resolution of the measurement [27]. Thus, the PLEmeasurement identifies the CQD absorption resonances thatresult in luminescence at 2.11 eV (2.30, 2.52, 2.55, 2.61, and2.75 eV—see Fig. 6 for more detail). It is important to notethat the PLE spectrum shown in the inset is very similar toanother published report, especially in terms of the fall-off inPLE intensity for increasing excitation energy [27].

Three spin recipes are used to deposit the as-synthesizedCdSe-CdS CQDs on GaAs substrates [28]: i) 0 revolutions perminute (rpm) (ramp rate rpm/s, spin speed rpm); ii)4000 rpm (ramp rate rpm/s, spin speed rpm,

Fig. 1. Room-temperature PL intensity as a function of detection energy for arange of excitation energies (2.57–3.82 eV) in as-synthesized CdSe-CdS CQDs.The corresponding PLE detection energy for the as-synthesized CQD PLE spec-trum (shown in the inset) is also noted.

spin time s); and iii) 8000 rpm (ramp rate rpm/s,spin speed rpm, spin time s). The epi-readyGaAs substrates from Wafer Technology feature differentdoping characteristics: semi-insulating (SI), n-type, and p-type.It is important to note that the 0 rpm spin recipe is equivalentto the direct adsorption of the CQD solution onto the GaAssubstrate surface.

III. STRUCTURAL CHARACTERIZATION

AFM images are used to characterize the GaAs substratesurface after spin-cast deposition of CQDs. As an example,Fig. 2(a), (b), and (c) shows CdSe-CdS CQDs spin cast on n-typeGaAs using the 0, 4000, and 8000 rpm recipes, respectively. Notethat spin-casting (4000 and 8000 rpm recipes) induces nearlyspherical clusters containing many CQDs distributed across theGaAs surface, whereas direct adsorption (0 rpm recipe) resultsin a more agglomerated, globular distribution of CQDs. Thespin-cast cluster sizes range from 1.6 to 10.7 m across the sixsamples and are orders of magnitude larger than a single CQD.The maximum height of the clusters is approximately 200 nm,which corresponds to 222 CQD monolayers (i.e., 0.89 nm forCdS shell CdSe core CdS shell). Note that the average size ofthe clusters depends on the spin recipe used in that the 4000 rpmrecipe yields larger clusters than the 8000 rpm recipe. Fig. 3shows histograms of the cluster diameter (assuming sphericalshape) for the 4000 and 8000 rpm spin recipes on SI, n-type, andp-type GaAs substrates. Each of these histograms is generatedfrom a single AFM image obtained 2 mm from the spin centerusing a 50 m scale. The interesting conclusion that can bedrawn from these histograms is that the surface distribution ofthe clusters depends on the doping characteristics of the GaAssubstrate much more than the spin recipe used. The clustershave the smallest diameters and narrowest size distribution on SIGaAs. The largest diameters and broadest size distribution occuron p-type GaAs, while n-type GaAs yields diameters and size

STIFF-ROBERTS et al.: SPIN-CAST DEPOSITION OF CdSe-CdS CORE-SHELL COLLOIDAL QUANTUM DOTS ON DOPED GaAs SUBSTRATES 415

Fig. 2. AFM images of CdSe-CdS core-shell CQDs spin-cast on n-type GaAsusing spin recipes: (a) 0 rpm; (b) 4000 rpm; and (c) 8000 rpm. The lateral scaleis 30 �m for (a) and 50 �m for (b) and (c). The approximate distance from thespin center is 2 mm.

distribution in the middle. It is important to note that while theAFM images of a single sample vary with distance from the spincenter, as shown in Fig. 4, these images do not vary at a givenposition. Therefore, a single measurement by AFM is sufficientto generate the structural information for each condition reportedherein.

Assuming these clusters have a spherical shape, the averagecluster size (cross-sectional diameter), the cluster surface den-sity, and the surface area fill factor (total cluster cross-sectionalarea/total AFM scan area) are determined from Fig. 3. Thecluster dependence on spin recipe and GaAs substrate doping isshown in Table I. Note that as the average cluster size increases,the cluster surface density decreases, and the cluster fill factorincreases. This is seen most clearly for the 4000 rpm spinrecipe, while the trends are less pronounced in the 8000 rpmfill factor.

The observed dependence of cluster surface coverage onthe GaAs substrate doping is indicative of charge interactionsbetween the CQDs and semiconductor material. This chargeinteraction is most likely due to the TOPO surface ligandsencapsulating the CQDs. It is also important to note that thepolarity of the CQD solvent could also affect the surfaceadsorption of CQDs on the GaAs substrate surface [29].

Fig. 3. Histograms (approximately 2 mm from the spin center) of CdSe-CdScore-shell CQD cluster sizes for SI, n-type, and p-type GaAs substrates using4000 and 8000 rpm spin recipes. The data is obtained from AFM images on a50 �m scale.

Further evidence for the substrate doping dependence ofcluster formation is provided by Fig. 4(a) and (b) in which theAFM-estimated cluster surface density (for 4000 and 8000 rpmrecipes) is shown as a function of distance from the spin centerfor n-type and p-type GaAs substrates, respectively. Each datapoint in Fig. 4(a) and (b) is determined from the adjacentAFM image. The most striking observation from Fig. 4 is that,for n-type GaAs, the cluster surface density increases for the4000 rpm spin recipe and decreases for the 8000 rpm spinrecipe as the distance from the spin center increases. Con-versely, for p-type GaAs, the cluster surface density decreasesfor the 4000 rpm spin recipe and increases for the 8000 rpmspin recipe as the distance from the spin center increases. Thus,by controlling substrate doping, it should be possible to usespin-casting to achieve a surface distribution of clusters witha fill factor sufficiently high to approach a thin film to enableoptoelectronic device active regions.


Fig. 4. CdSe-CdS core-shell CQD cluster surface density as function of dis-tance from spin center for 4000 and 8000 rpm spin recipes on: (a) n-type and(b) p-type GaAs substrates. The insets at each data point show the correspondingAFM images (height scale �200 nm; lateral scale � 50 �m).

TABLE ISTRUCTURAL CHARACTERISTICS OF CdSe-CdS CQD CLUSTERS ON DOPED

GaAs SUBSTRATES DETERMINED BY ATOMIC FORCE MICROSCOPY 2 mmFROM SPIN CENTER

IV. OPTICAL CHARACTERIZATION

The PL and PLE spectroscopy measurements discussed inthe remainder of this letter aim to correlate spin recipe andGaAs substrate doping with CQD optical response. Directly or

indirectly, the ability to control the optical properties of CQDsspin-cast on GaAs substrates through doping concentrationwill enable optoelectronic device design. This optical char-acterization provides further evidence of charge interactionsbetween the CQDs and GaAs substrate, as well as informationabout the influence of clustering. The PL measurements fordifferent excitation energies are analyzed by considering thefull-width half-maximum (FWHM) linewidth and the peakenergy for as-synthesized CQDs in solution and spin-castCQDs on n-type and p-type GaAs substrates. Fig. 5(a) and(b) shows the room-temperature PL FWHM linewidth as afunction of excitation energy for spin-cast CQDs on n-type andp-type GaAs, respectively; while Fig. 5(c) and (d) shows theroom-temperature PL peak energy as a function of excitationenergy for spin-cast CQDs on n-type and p-type GaAs, respec-tively (0, 4000, and 8000 rpm spin recipes). The PL FWHMlinewidth and peak energy curves for as-synthesized CQDs insolution are included for comparison.

First, consider the influence of spin-casting on the PL spectraby comparing the as-synthesized and spin-cast CQDs. Notethat for excitation energy near 2.57 eV, there is a sharp decreasein PL FWHM linewidth for all samples. This is due to selectivephotoexcitation of only the largest CQDs, i.e., those thatexhibit the smallest band-to-band energy gap. This selectivephotoexcitation at the lower excitation energies is accompaniedby a slight red-shift in PL peak energy for the as-synthesizedCQDs. Also, notice that for as-synthesized CQDs, the FWHMlinewidth maximum is reached near an excitation energy nearof 2.6 eV and decreases for higher excitation energies. This isa consequence of the monodispersity of the CQDs: photoex-citation efficiency decreases and linewidth narrows at higherexcitation energies because there are fewer CQDs small enoughto absorb these photons. In contrast, note that for spin-castsamples the FWHM linewidths remain relatively constant withincreasing excitation energy. This is a consequence of photoex-citation in the GaAs substrate: charge transfer is occurring fromthe GaAs, causing the same distribution of CQDs to exhibitluminescence with steady-state efficiency as the excitationenergy increases. In terms of the PL peak energy, the spin-castCQDs on both n-type and p-type GaAs substrates exhibit peakenergy red-shifted 15–27 meV compared to the as-synthesizedCQDs in solution. This red-shift is most likely due to chargeinteractions with the GaAs substrate.

To determine the influence of the GaAs substrate doping,consider the PL FWHM linewidths and peak energies on n-typeand p-type GaAs. There is very little significant differenceamong the PL characteristics of most samples due to thesubstrate doping type. For example, the PL FWHM linewidthand peak energies for the 4000 and 8000 rpm spin recipes areessentially equivalent on both types of GaAs substrates. How-ever, there is one important exception: the 0 rpm spin recipe(direct adsorption) on p-type GaAs. The PL FWHM linewidthsof this sample are much larger and grow more dramaticallywith excitation energy than those for the other samples on n- orp-type GaAs. Further, the PL peak energy for the 0 rpm sampleon p-type GaAs is red-shifted 10 meV such that it can bedifferentiated from the other spin-cast CQD samples on both n-and p-type GaAs substrates. As in the case of the cluster surface


Fig. 5. Room-temperature PL characteristics for CdSe-CdS core-shell CQDs deposited using different spin-cast recipes: FWHM linewidth versus excitation en-ergy on (a) n-type and (b) p-type GaAs substrates; and peak energy versus excitation energy on (c) n-type and (d) p-type GaAs substrates. The PL characteristicsfor as-synthesized CQDs are included in each graph for comparison. The experimental uncertainty is �1 meV.

properties for spin-cast CQDs, the interaction of the TOPOsurface ligands with the p-type GaAs substrate is the likelyreason for this behavior in the 0 rpm sample. This proposedcharge interaction results from the transfer of hot holes inthe p-type GaAs substrate, to the TOPO surface ligands, andfinally to surface states of the CdSe-CdS CQDs. The hot holesthat experience charge transfer from the GaAs substrate to theCQDs are more likely to occupy surface states as opposed toconfined states since CdSe-CdS CQDs are electron acceptors(i.e., they are p-type) [23], [30]. As these excited carriers in thesurface states of the CQD experience radiative relaxation, theycontribute to a significantly broadened PL FWHM linewidth.This idea is supported by the dramatic reduction of the PLFWHM linewidth for spin-cast CQDs using the 4000 and8000 rpm recipes on p-type GaAs since these samples haveless TOPO material remaining on the surface as a result ofclustering.

Finally, Fig. 6 shows the room-temperature, normalized PLEintensity as a function of excitation energy (detected at thesame 2.11 eV energy denoted in Fig. 1 for the as-synthesized

CQD spectrum). It is important to note that the PLE spectra forspin-cast CdSe-CdS CQDs have been normalized according tothe calculated fill factor shown in Table I for SI, n-type, andp-type GaAs substrates using the 4000 rpm spin recipe. Thefill factor values are indicated in parentheses in the legend forFig. 6. More specifically, the p-type GaAs had the largest fillfactor (21.1%), and the SI and n-type GaAs samples were scaledaccordingly (i.e., SI GaAs scale factorand n-type GaAs scale factor ). Thepurpose of this normalization is to elucidate the two mech-anisms that contribute to the PLE intensity in the spin-castCQD samples: i) direct absorption by individual CQDs andii) charge transfer of photoexcited, hot carriers in GaAs

eV bandgap excitation energy to confined energylevels in the CQDs. Thus, the first mechanism depends on theamount of material present (cluster fill factor), and the secondmechanism depends on the efficiency of charge transfer fromthe GaAs substrate to the CQD confined levels.

Therefore, the fill factor normalization demonstrates thatdespite having the largest cluster fill factor, p-type GaAs ex-


Fig. 6. PLE spectra as a function of excitation energy for CdSe-CdS core-shellCQDs spin-cast on SI, n-type, and p-type GaAs substrates using the 4000 rpmspin recipe. The PLE spectrum for as-synthesized CQDs in solution is includedfor comparison.

hibits the smallest PLE intensity, thereby indicating that chargetransfer is least efficient on these substrates. This result agreesvery well with the optical characterization discussed previously;i.e., p-type GaAs exhibits charge transfer to CQD surface statesvia the TOPO surface ligands, thereby contributing to broaderlinewidths in the PL spectrum for the 0 rpm spin recipe. Incontrast, SI and n-type GaAs substrates experience efficientcharge transfer directly to confined energy levels of the CQDs,as demonstrated by the larger PLE intensities. These conclu-sions are reasonable given that CdSe-CdS CQDs are electronacceptors. In addition, these charge transfer mechanisms areresponsible for differences in the PLE spectrum of as-synthe-sized and spin-cast CQDs for increasing excitation energies.While the as-synthesized CQD PLE intensity decreases withincreasing excitation energy, the spin-cast CQDs exhibit theopposite behavior on SI, n-type, and p-type GaAs substrates.Thus, the as-synthesized PLE spectrum shows CQD directabsorption only, while the spin-cast PLE spectra demonstrateadditional charge interactions due to photoexcitation of hot car-riers in GaAs and subsequent charge transfer to confined statesin the CdSe-CdS CQDs such that the luminescence at 2.11 eVincreases with increasing excitation energy. It is importantto note that there is also a significant difference between thePLE intensities of the spin-cast CQDs on SI and n-type GaAssubstrates, demonstrating that charge transfer of hot carriersis much more efficient on SI GaAs. While the origin of thisincreased efficiency is not yet understood, the important pointis that a clear trend emerges in the optical response of spin-castCQDs as the doping of the GaAs substrate is varied from mostpositive to most negative. This correlation between the sub-strate doping concentration/type and the CQD optical responsewill enable device design, and further studies are requiredto better quantify these dependencies. A final observation isthat the spin-cast induced clustering observed from the AFMimages does not change the absorption resonances measuredfrom individual CQDs. Thus, the resonant peaks identifiedfor the as-synthesized CQDs are also present in the spectra

for the spin-cast CQDs on all three types of GaAs substrates,indicating that even though there is significant clustering on theGaAs surface, the structural, optical, and quantum mechanicalintegrity of individual CQDs is maintained.

V. CONCLUSION

To conclude, the detailed and systematic structural and op-tical characterizations of spin-cast CdSe-CdS CQDs on dopedGaAs substrates presented in this letter provide a comprehen-sive analysis of the material system and enable the observationof an important physical phenomenon for the first time; i.e.,the dependence of the spin-cast induced CQD cluster surfacedistribution on the interaction of TOPO surface ligands withdoped GaAs substrates. In addition, these TOPO surface lig-ands are shown to be instrumental in the charge transfer of hotholes from the GaAs substrate to CQD surface states. Thus,while the TOPO surface ligands are often neglected in theo-retical treatments of CQDs, they are very important to deter-mining the optoelectronic properties of CQDs when embeddedin semiconductor matrices for device applications [22], [31].The comparison of measured spin-cast CQD properties to thoseof as-synthesized CQDs also demonstrates that the hypothesesof this paper are true, yet realized in unexpected ways. First,while it is possible to control the surface density and uniformityof spin-cast CQDs, the structural properties (i.e., spin-cast in-duced clustering) are much more dependent on substrate dopingthan spin recipe. Second, while the GaAs substrate doping typedoes influence charge transfer processes, it does not alter the ab-sorption resonances of individual CQDs, but rather the chargetransfer efficiency and the types of CQD states that are occu-pied by the hot carriers originating in GaAs. These results haveimportant implications for device operation, and should enableunique device design by controlling the doping concentration ofthe semiconductor substrate or an epitaxial layer.

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Adrienne D. Stiff-Roberts (M’99) received the B.S.degree in physics from Spelman College, Atlanta,GA, in 1999, the B.E.E. degree in electrical engi-neering from the Georgia Institute of Technology,Atlanta, in 1999, and the M.S.E. degree in elec-trical engineering and the Ph.D. degree in appliedphysics from the University of Michigan, AnnArbor, in 2001 and 2004, respectively, where sheinvestigated high-temperature quantum dot infraredphotodetectors.

She is an Assistant Professor in the Departmentof Electrical and Computer Engineering, Duke University, Durham, NC. Herresearch interests encompass the epitaxial growth and characterization ofquantum-confined semiconductor materials; the synthesis and characterizationof hybrid nanomaterial thin films; and the design, fabrication, and characteriza-tion of optoelectronic and photonic devices, especially in the infrared regime.

Prof. Stiff-Roberts received the David and Lucile Packard Foundation Grad-uate Scholars Fellowship and the AT&T Labs Fellowship Program Grant from1999 to 2004. She is a member of Phi Beta Kappa and Sigma Pi Sigma, and sheis a recipient of the National Science Foundation CAREER Award in 2006.

Wanming Zhang received the B.S. degree in mate-rials science and engineering from Tianjin Univer-sity, Tianjin, China, in 1991 and the M.S. degree inmaterials science and engineering from the Instituteof Metal Research (IMR), Chinese Academy of Sci-ences, Shenyang, in 1995. He is currently workingtoward the Ph.D. degree in the Department of Elec-trical and Computer Engineering, Duke University,Durham, NC.

Before joining Duke, he worked at IMR as a Re-search Fellow and Northwestern Polytechnical Uni-

versity, Xi’an, China, as a Research Scientist. During this period he focused onadvanced aluminum alloys and single crystal growth techniques of advanced in-termetallic compounds. He has published nine papers and holds one patent inChina. His academic research interests include the design and molecular beamepitaxy growth of advanced semiconductor heterostructures, optimization ofepi-layer interfaces, and semiconductor nanocrystals.

Jian Xu, photograph and biography not available at the time of publication.

Hongying Peng received the B.S. degree in materialsscience and engineering from Tianjin University,China, in 1991, the M.S. degree in materials sci-ence and engineering from Dalian University ofTechnology, China, in 1994, and the Ph.D degree inapplied physics from the Institute of Metal Research,Chinese Academy of Sciences, in 1997.

She was a Postdoctoral Associate in the Center ofSuper-Diamond and Advanced films and the Depart-ment of Physics and Materials Science, City Univer-sity of Hong Kong, from 1998 to 2000. She was a

Research Scientist in Institute for Shock Physics, Washington State University,between 2000 and 2003, and was associated with the Department of Physics,Duke University, in 2003–2005.. She joined the GE Global Research Center,Niskayuna, NY, in 2005.

Dr. Peng received the National Research Council Associateship Award in2003.


Henry O. Everitt received the B.S. degree in physicsand mathematics and the M.A. and Ph.D. degrees inphysics from Duke University, Durham, NC, in 1985,1987, and 1990, respectively.

He joined the Army Research Office as a programmanager in 1991 where he managed a program incondensed matter physics, concentrating on pho-tonic band engineering, nanoscience, and quantuminformation. He also became an adjunct professor ofphysics at Duke University in 1992 and subsequentlywas appointed as an adjunct professor of Electrical

and Computer Engineering. By 2001 he was chief of the ARO Physics Division

and became ARO chief scientist in 2004. In 2005 he became senior researchscientist at the Army’s Aviation and Missile Research, Development, and Engi-neering Center at Redstone Arsenal, AL, where he serves as chief scientist of theWeapons Sciences Directorate. His main research interests include the ultrafastoptical characterization of wide bandgap semiconductor heterostructures andnanostructures and the development of THz spectroscopic imaging techniques.

Dr. Everitt is a Fellow of the Optical Society of America and an emeritusFellow of the Army Research Laboratory. He is a member of the AmericanPhysical Society, the American Association for the Advancement of Science,and Sigma Xi. He has served on numerous executive planning committees ofseveral scientific societies and has organized special issues of journals for IEEEand OSA.

Documents

Spin-Cast Deposition of CdSe-CdS Core-Shell Colloidal Quantum Dots on Doped GaAs Substrates: Structural and Optical Characterization