Diversity of sub-bandgap states in lead-sulfide nanocrystals: real-space spectroscopy and mapping at the atomic-scale—Christian F. Gervasi, Dmitry A. Kislitsyn, Thomas L. Allen, Jason D. Hackley, Ryuichiro Maruyamaa, and George V. Nazin 

Colloidal semiconductor nanocrystals are a promising class of technological materials with optoelectronic properties controllable through quantum-confinement effects. It is difficult to control the impact of the nanocrystal surface structure on the photophysics and electron transport in nanocrystal-based materials is difficult. In particular, the presence of surface defects and irregularities can result in the formation of localized sub-bandgap states that can dramatically affect the dynamics of charge carriers and electronic excitations. Here, scanning tunneling spectroscopy (STS) is used to investigate, in real space, sub-bandgap states in individual ligand-free PbS nanocrystals. This study provides insight into the mechanisms of sub-bandgap state formation that, in a modified form, are likely to be applicable to ligand-passivated nanocrystal surfaces, where steric hindrance between ligands can result in under-coordination of surface atoms. The published article (Nanoscale, Issue 46, 2015, pp. 19732-19742) is available here.

Synthesis of Colloidal PbSe Nanoparticles Using a Microwave-Assisted Segmented Flow Reactor—Eric B. Hostetler, Ki-Joong Kim, Richard P. Oleksak, Robert C. Fitzmorris, Daniel A. Peterson, Padmavathi Chandran, Chih-Hung Chang, Brian K. Paul, David M. Schut, and Gregory S. Herman

This study (Materials Letters, Elsevier, Volume 128, 1 August 2014, Pages 54–59) evaluates the effect of the nucleation temperature on PbSe nanoparticle size distribution, crystallographic structure, particle shape, and particle composition. It was found that nucleation of Pb-rich species occurs in the microwave reaction zone, while PbSe nanoparticles form in the growth zone. This is an author’s peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by Elsevier and can be found here.

Spatial Mapping of Sub-Bandgap States Induced by Local Nonstoichiometry in Individual Lead Sulfide Nanocrystals—D. A. Kislitsyn, C. F. Gervasi, T. Allen, P. K. B. Palomaki, J. D. Hackley, R. Maruyama, and G. V. Nazin

This paper (Journal of Physical Chemistry Letters, vol. 5, pp 3701–3707, Oct. 2014) explores the electronic structure of individual lead sulfide nanocrystals, which have applications in solar cells, photodetectors, and light-emitting devices.  Professor Nazin’s specialized scanning tunneling microscopy system allows scientists, for the first time, to spatially map the electronic states present in a single nanocrystal, providing clues for further tuning of the nanocrystal properties.  The observations made studying VoxtelNano’s lead sulfide quantum dots have already led to an improved understanding of devices currently in development at VoxtelNano. The published article is available here.

Pixelated Detector with Photon Address Event Driven Time Stamping and Correlation—George M. Williams Jr., Jehyuk Rhee, Adam Lee, and Stephen D. Kevan

In the paper, we present the design, manufacture, and test results of an asynchronous event-driven address time-stamped (EDATS) pixelated array detector, operational over correlation time spans ranging from less than 10−6 to greater than 104 seconds. The pixelated EDATS detector was designed to measure the equilibrium density fluctuations on a nanometerlength scale using small angle x-ray scattering in x-ray photon correlation spectroscopy (XPCS) experiments. The detector sensor chip assembly (SCA) includes a custom readout integrated circuit (ROIC), hybridized to a silicon photodiode array, optimized for 500 eV to 2000 eV x-ray photons. The detector is shown to be capable of handling x-ray photon event rates of 100 million x-ray events per second, with less than 85 nanoseconds timing jitter per event.

Discrimination of Photon- and Dark-Initiated Signals in Multiple Gain Stage APD Photoreceivers—GM Williams and AS Huntington with U of New Mexico, accepted for publication

We demonstrate the ability of linear mode single carrier multiplication (SCM) avalanche photodiode (APD) -based optical receivers to discriminate single-photon-initiated avalanche events from dark-current-initiated events. Because of their random spatial origin in discrete regions of the depletion region, in the SCM APD the dark-generated carriers multiply differently than the photon-generated carriers. This causes different count distributions and necessitates different statistical descriptions of the signal contributions from photon- and dark-originating impulse responses. To include dark carriers in the performance models of the SCM APD, we considered the influence of the spatial origin of the ionization chains on a receiver’s noise performance over the times the optical pulse is integrated by the receiver’s decision circuits. We compare instantaneous (time-resolved) numeric and pseudo-DC analytical models to measured SCM APD data. It is shown that it is necessary to consider both the distribution of spatial origin and the instantaneous properties of the ionization chains to describe statistically an SCM APD receiver. The ability of SCM APD receivers to discriminate single photon events from single dark events is demonstrated, and the effective gain and excess noise contributions of the light- and dark-initiated avalanche events and their influence on receiver sensitivity and signal-to-noise characteristics is shown.

Multi-gain-stage InGaAs Avalanche Photodiode with Enhanced Gain and Reduced Excess NoiseGM Williams, M Compton, and AS Huntington with U of New Mexico, submitted for publication

We report the design, fabrication, and test of an InGaAs avalanche photodiode (APD) for 950-1650 nm wavelength sensing applications. The APD is grown by molecular beam epitaxy on InP substrates from lattice-matched InGaAs and InAlAs alloys. Avalanche multiplication inside the APD occurs in a series of asymmetric gain stages whose layer ordering acts to enhance the rate of electron-initiated impact ionization and to suppress the rate of hole-initiated ionization when operated at low gain. The multiplication stages are cascaded in series, interposed with carrier relaxation layers in which the electric field is low, preventing avalanche feedback between stages. These measures result in much lower excess multiplication noise – and stable linear-mode operation at much higher avalanche gain – than is characteristic of APDs fabricated from the same semiconductor alloys in bulk. The noise suppression mechanism is analyzed by simulations of impact ionization spatial distribution and gain statistics, and measurements on APDs implementing the design are presented. The devices employing this design are demonstrated to operate at linear-mode gain in excess of 6,000 without avalanche breakdown. Excess noise characterized by an effective impact ionization rate ratio below 0.04 were measured at gains over 1,000.

Non-local Model for the Spatial Distribution of Impact Ionization Events in Avalanche PhotodiodesGM Williams and AS Huntington with U of New Mexico, submitted for publication

We report an extension of the analytical Dead Space Multiplication Theory [IEEE Trans. Electr. Dev., vol. 39, pp. 546-552, 1992]that provides the means to analytically determine the spatial distribution of electron and hole impact-ionization events in an arbitrarily specified heterojunction multiplication region. The model can be used to understand the role of dead space in regularizing the locations of impact ionization. It can also be utilized to analyze, design and optimize new generations of ultra-low noise, multi-staged gain avalanche photodiodes based upon judiciously energizing and relaxing carriers to enhance electron impact ionizations and suppress hole impact ionizations.

Time resolved gain and excess noise properties of InGaAs/InAlAs avalanche photodiodes with cascaded discrete gain layer multiplication regions – GM Williams and AS Huntington with U of New Mexico, 2012

Copyright 2013 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in the Journal of Applied Physics, vol. 113, Mar. 2013 and may be found at

The paper shows that the traditional McIntyre equation often used to model APD performance is inadequate for describing photoreceiver performance. The paper also shows the performance benefits of Voxtel’s SCM APD technology compared to conventional APD technology.

Mesa-isolated InGaAs avalanche photodiode damage by ionizing radiation – AS Huntington and MA Compton with International Photonics Consultants Inc., 2011

This paper assesses the sensitivity of certain avalanche photodiodes (APDs) to a total ionizing dose of 200 krad(Si). Low-excess-noise InGaAs APDs with a multi-stage avalanche gain region were fabricated from epitaxial material by etching detector mesas and encapsulating the etched mesas under bisbenzocyclobutene (BCB) resin. The APDs were irradiated with Co-60 gamma-rays and tested. To assess the response of the design to ionizing radiation, 96 identical 20-μm-diameter APDs were characterized. The APDs were not under bias during irradiation. Damage to the APDs was characterized by measuring the change in room temperature dark current following irradiation, at a reverse bias for which the average avalanche gain is M = 10. No significant increase of dark current was observed following gamma irradiation; the average increase was 5% and the standard deviation for the measurement was 10%.

Limitations of Geiger-Mode Arrays for Flash LADAR Applications – GM Williams, 2010

It is shown through physics-based Monte Carlo simulations of avalanche photodiode (APD) LADAR receivers that
under typical operating scenarios, Geiger-mode APD (GmAPD) flash LADAR receivers may often be ineffective…

Theory for Spatial Distribution of Impact-Ionization Events in Avalanche Photodiodes – AS Huntington and GM Williams with U of New Mexico, 2012

In this paper we report an extension of the Dead Space Multiplication Theory (DSMT) that enables determining the spatial distribution of the impact ionizations for arbitrary heterojunction multiplication regions.

Probabilistic analysis of Linear Mode vs Geiger Mode APD FPAs– AS Huntington and GM Williams 2008

We analyze the potential benefits of a LADAR receiver based on Linear Mode SPADs, which include: 1) the ability to obtain range information from more than one object in a pixel’s instantaneous-field-of-view (IFOV), 2) a lower false alarm rate, 3) the ability to detect targets behind debris, 4) an advantage in the endgame, when stronger reflected signals allow dark current rejection via thresholding, and 5) the ability to record signal intensity, which can be used to increase kill efficiency. As expected, multiple laser shots of the same scene improves the target detection probability.

Characterization of HgCdTe MWIR Back-Illuminated Electron-Initiated Avalanche Photodiodes– M. Compton and G. M. Williams with M. Reine at all (BAE), 2007

This paper reports new characterization data for large-area (250×250 μm²) back-illuminated planar non-p HgCdTe electron-initiated avalanche photodiodes (e-APDs).We report data for F(M)≈1 for gain up to 150 at 196 K. We describe the features of the abrupt breakdown phenomenon seen in most of our devices at high reverse bias voltages.

Improved breakdown model for estimating dark count rate in avalanche photodiodes with InP and InAlAs multiplication layers_ AS. Huntington, MA Compton, GM Williams

 We present an improved method for estimating the dark count rate of single-photon-sensitive avalanche photodiodes (SPADs) with either InP or InAlAs multiplication layers…In combination with local models of dark carrier generation, our technique can provide more realistic estimates of dark count rate than are obtained by multiplying the primary dark current by a single junction breakdown probability, or by assuming constant electric fields in the multiplication layer.

Linear-Mode Avalanche Photo-Diode Detectors with a Quasi- Deterministic Gain Component: Statistical Model Studies – – GM Williams and AS Huntington with D. Youmans (SPARTA)

In this paper we examine the ROC (Pdetection vs PFalseAlarm) statistics of these single photon APDs as a function of the quasi-deterministic mean gain and standard deviation for an rms ROIC (readout integrated circuit) noise level of 25e-. Single photo-electron and multiple photo-electron detection statistics are also examined for predicting a ROC. Measured linear-mode APD data are also presented.

 GHz-rate Single Photon Sensitive Linear Mode APD Receivers – GM Williams 2008

 We report the design, fabrication, and test of a new InGaAs avalanche photodiode (APD) for short-wavelength infrared (SWIR) sensing applications at 950-1650 nm….High rate single photon counting at 1064 nm was demonstrated with multiple 10-stage APDs operated below their breakdown voltage, using a commercial 2-GHz transimpedance amplifier (TIA) chip. Single photon detection efficiencies as high as 70% were measured for signal photon rates of 50 MHz.

High-speed photon counting with linear-mode APD Receivers – GM Williams, MA Compton, AS Huntington

HgCdTe and InGaAs linear mode avalanche photodiodes (APDs) were fabricated and tested for properties suitable for high speed photon counting when integrated with commercially available 2-GHz resistive transimpedance amplifiers (RTIA). The 2.71-μm, 100-μm diameter-HgCdTe APDs were fabricated in using a n+/p vertical carrier transport architecture designed to reduce carrier drift time and facilitate high speed operation. At 215K, a gain of 100 was measured with an excess noise of 2.5…The InGaAs/InAlAs APDs were integrated into receivers consist of a multi-gain-stage APD coupled to a commercial 2-GHz resistive transimpedance amplifier (RTIA) and are operated as thresholded photon-counters. At a linear gain of M=1800, a single photon detection efficiency greater than 85% was measured at a maximum count rate of 70 MHz

Linear-mode single-photon APD detectors – AS Huntington, MA Compton, GM Williams – 2008

We review the performance thresholds required to achieve linear-mode photon counting, and present measurements from two APD designs that meet the gain and noise requirements. The first design is a previously-reported vertical-junction, electron-avalanche HgCdTe device fabricated from 4.06-µm-cutoff liquid phase epitaxy (LPE)-grown material. These HgCdTe APDs have an excess noise factor of approximately F~1 at a gain of M=150 when measured at 196 K. The second design is a novel InAlAs/InGaAs structure grown by molecular beam epitaxy (MBE) entirely from alloys lattice-matched to InP. The maximum gain found for this new design was as high as M=2000 at 235 K…

Single-photon-sensitive linear-mode APD ladar receiver developments -GM Williams, MA Compton, AS Huntington

New measurements are presented for multi-stage InGaAs avalanche photodiodes (APDs) which have the potential to perform GHz-rate single photon counting in linear mode. No increase in dark current was measured for an 11-device sample of 5-stage APDs following 717 hours of accelerated aging under bias at 50°C, during an initial lifetime study. … Good agreement between calculated non-ionizing energy loss (NIEL) and observed damage was found for the low-energy protons at fluences of 1010 and 1011 cm-2. A NIEL calculation successfully predicted the damage observed following a 5×1010 cm-2 dose of 63.5-MeV protons by extrapolating from 2 MeV data, which suggests that displacement damage is the dominant mechanism.

Nanocrystal Sensitized Photovoltaics and Photodetectors With Performance Enhanced Using Ligand Engineering -DM Schut, GM Williams, SA Arteaga, TL Allen, T Novet – 2010

A new particle, the Janus-II nanoparticles, developed using ‘charge-donating’ and ‘charge-withdrawing’ ligands distributed over opposite surfaces of the nanocrystal, is described. The polarizing ligands of the Janus-II nanoparticle form a degeneracy-splitting dipole, which reduces the overlap integral between excitonic states, and thus reduces the probability of carrier recombination, allowing carrier extraction to take place more efficiently. This is shown to allow increased photodetection efficiencies and to allow the capture of multiple exciton events in working photodetectors.

Optically Coded Nanocrystal Taggants and Optical Frequency IDs – GM Williams, T Allen, D, Dupoy, T. Novet, D, Schut – 2010

A series of nanocrystal and nanocrystal quantum dot taggant technologies were developed for covertly tagging and tracking objects of interest. Homogeneous and heterogeneous nanocrystal taggant designs were developed and optimized for ultraviolet through infrared emissions, utilizing either Dexter energy transfer or Förster resonant energy transfer (FRET) between specific absorbing and emitting functionalities. The conversion efficiency, target-specific identification, and adhesion properties of the taggants were engineered by means of various surface ligand chemistries

Solution Processible Nanoparticle Thermoelectric Materials – N. Nguten, GM Williams, T Allen, D. Shut, etal – 2011

The fabrication of stoichiometric (BixSb1- x)2Te3 thermoelectric films comprised of nanostructured building blocks were fabricated using solution processing compatible methods…A Seebeck coefficient was measured to be -235 μVK-1 for the n-type Bi2Te3 films, and 262 μVK -1 for the p-type (Bi0.25S0.75)2Te3 films. Although limited by high resistance, due to cracking of the films, ZT was estimated to be between 0.8 and 1.69 for the n-type films and an order of magnitude lower for the p-type films.

Dual-Band MWIR/LWIR Radiometer for Absolute Temperature Measurements – GM Williams, A Barter 2007

A Dual-band Radiometer (DBR) has been developed to accurately measure temperature at long ranges. Key to the DBR is a dual-band, quantum well infrared photodetector (QWIP) focal plane array (FPA) that integrates within each pixel both mid-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) spectral sensitivity. A vertically-integrated, two-color FPA eliminates with inter-band optical distortions, temperature-induced alignment errors, and improves radiometric measurement accuracy (Won Best Paper of Conference)

 Nanoparticle Security Coatings and Inks Benefit Brand Owners – GM Williams 2011

Counterfeit products have been a problem in commercial market sectors for many years, and worldwide industry loses large amounts to counterfeiters. It is estimated that the total global economic value of counterfeit and pirated products is as much as $650 billion every year – more than 5% of world trade. The U.S. alone is estimated to annually import $50 billion in pirated and counterfeit products.

Electron bombarded back-illuminated CCD sensors for low light level imaging applications – GM Williams etal. 1993

…electron bombarded CCD (EBCCD) sensors were designed and fabricated. Experiments demonstrated the EBCCD’s sensitivity and contrast resolution superior to conventional intensified CCD (ICCD) approaches. Low light level signal to noise (STN) and contrast transfer function (CTF) data are presented. A model is derived that describes the performance of the EBCCD and the back-illuminated CCD relative to conventional approaches to nighttime imaging. A
design and simulated performance of a video rate 2/3 inch, back-illuminated, electron bombarded CCD currently under development for low light imaging applications is also described.

PbS Nanocrystals Functionalized with a Short-Chain, Ionic, Dithiol Ligand – AS Stonas with U of Oregon (IS Moody) 2008

Ligand exchange from oleic acid-capped PbS nanocrystals was used to prepare 2,3-dimercaptopropanesulfonate (DT) functionalized, water-soluble PbS nanocrystals….Due to the compact size of the DT ligand, thin lms of PbS-DT nanocrystals were observed to exhibit much greater photoconductivity than PbS nanocrystals capped with the longer chain oleic acid ligand. Responsivities of greater than 1 A/W in the near infrared were achieved with thin lms of PbS-DT NCs on interdigitated array electrodes.


We present design and performance data for a high speed telecom-band (1.3 μm) single-photon-sensitive receiver based on a new class of multi-stage InGaAs avalanche photodiode (APD) operated in the proportional mode (linear mode). Peak photon detection efficiency of 70% was measured at 1.064 μm. The multi-stage APDs demonstrated can be operated with a linear gain in excess of M = 8000, and have silicon-like multiplication noise characterized by an effective ionization coefficient ratio of k = 0.03 out to M = 1000.

HgCdTe MWIR Back-Illuminated Electron-Initiated Avalanche Photodiode Arrays – GM Williams with M. Reine etal. 2006

This paper reports performance data for back-illuminated planar n-on-p HgCdTe electron-initiated avalanche photodiode (e-APD) 4×4 arrays with large-area unit cells (250×250 μm²).Spot scan data show that both the V=0 response and the gain at V=-5.0 V are quite uniform spatially over the large junction area. To the best of our knowledge, these are the first spot scan data for avalanche gain ever reported for HgCdTe e-APDs. Capacitance versus voltage data are consistent with an ideal abrupt junction having a donor concentration equal to the indium counterdoping concentration in the as-grown LPE film. 

PbS Nanocrystals Functionalized with a Short-Chain, Ionic, Dithiol LigandAR Stonas and U of Oregon 2010

Ligand exchange from oleic acid-capped PbS nanocrystals was used to prepare sodium 2,3-dimercaptopropanesulfonate (DT) functionalized, water-soluble PbS nanocrystals. The greater stability of the PbS-DT nanocrystals is argued to be due to the formation of oligodisulfides that remain partially bound to the nanocrystal surface thereby inhibiting precipitation. Due to the compact size of the DT ligand, thin films of PbS-DT nanocrystals were observed to exhibit much greater photoconductivity than PbS nanocrystals capped with the longer chain oleic acid ligand. Responsivities of greater than 1 A/W in the near-infrared were achieved with thin films of PbS-DT NCs on interdigitated array electrodes.

Multiple Exciton Collection in a Sensitized Photovoltaic SystemTE Novet and U of Wyoming – 2010

We report on the first ever capture of  multiple exciton generated (MEG) carriers in a working OPV device. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO2 facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.

Investigation of ligand effects on exciton  recombination in PbS nanoparticlesTE Novet and U of Oregon – 2012

Multiple exciton generation (MEG) and exciton recombination were studied by femtosecond transient absorption spectroscopy in PbS nanoparticles (NPs) capped with oleic acid (PbS–OLA) and 2,3-dimercaptopropane sulfonate (PbS– DMPS) ligands. Analysis of the transient absorption data showed that the PbS–DMPS nanoparticles exhibit increased rates
of multi- and single-exciton recombination compared with the PbS–OLA nanoparticles; however, the MEG yield for both sets of particles was the same within experimental error. Overall, this study showed that the dynamics of MEG can be modified by changing the NP ligand shell, a result that may be useful in the development of NP-based thin film solar devices.