Kevin R. Lantz

Effect of donor-complex-defect-induced dipole field on InAs/GaAs quantum dot infrared photodetector activation energy

In order to understand dopant incorporation in quantum dot infrared photodetectors, three quantum dot (QD) Schottky diodes (undoped, delta doped, and modulation doped) have been investigated. Donor-complex-defect (DX) centers have been observed by photocapacitance quenching in the doped diodes only. When the applied bias increases, the doped samples show a rapid increase in dark current and a resulting dramatic decrease in QD activation energy. The activation energy reduction could be related to a dipole field between positively charged DX centers and electrons in QDs. A transport mechanism is proposed to explain the observed activation energy bias dependence in the doped samples.

Room-temperature, mid-infrared photodetection in colloidal quantum dot/conjugated polymer hybrid nanocomposites: a new approach to quantum dot infrared photodetectors

A unique and distinct approach to unipolar, intraband transitions appropriate for room-temperature, mid- and long-wave-infrared (IR) photodetection is to use active regions comprising colloidal quantum dots (CQDs) synthesized by inorganic chemistry embedded in conjugated polymers. The polymer not only enhances quantum confinement of and electron localization in CQDs, but it also assists in the conduction of electrons photogenerated by the absorption of IR light. Preliminary demonstrations of intraband, mid-wave IR absorption and Fourier transform IR (FTIR) spectral response at room temperature in CdSe/MEH-CN-PPV hybrid nanocomposite thin films are promising, yet the inherent lack of control over hybrid nanocomposite morphology due to solution-based deposition is an inherent challenge. Matrix-assisted pulsed laser evaporation is a vacuum-based deposition technique that could address this challenge by providing more homogeneous distributions of CQDs in hybrid nanocomposite thin films.

Calculation of Intraband Absorption Coefficients in Organic/Inorganic Nanocomposites: Effects of Colloidal Quantum Dot Surface Ligand and Dot Size

Hybrid nanocomposite thin films composed of inorganic colloidal quantum dots (CQDs) embedded in an organic conjugated polymer have shown promise as a method for room-temperature infrared detection due to the 3-D confinement of the CQD. The CQDs are coated with a surface ligand material which is comprised of short, organic molecules that prevent the CQDs from aggregating when placed in solution. These surface ligand materials behave as a thin, insulating layer that has been shown to prevent efficient transfer of excited carriers into and out of the CQD. Therefore, it is important to understand the effect that the surface ligand material has on the optical properties of the nanocomposite materials in order to design more efficient hybrid nanocomposite optoelectronic devices. In this paper, we calculate the infrared, intraband absorption coefficient for CQDs in a nanocomposite thin film. The model is verified by comparing the calculated absorption coefficient spectrum to a measured FTIR absorbance spectrum for a specific hybrid nanocomposite material system. Importantly, the CQD surface ligand is included in the model explicitly, which enables the selection of the surface ligand material to be considered as a design parameter for infrared, intraband absorption in hybrid nanocomposites. In addition, the CQD average size is also treated as design parameter in order to tune the infrared, intraband absorption coefficient in hybrid nanocomposites.

Comparison of conjugated polymer deposition techniques by photoluminescence spectroscopy

The effects of various deposition techniques on the photoluminescence spectra of the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyanovinylene) phenylene] (MEH-CN-PPV) are investigated. Photoluminescence spectroscopy provides insight to the internal morphology of organic thin films through the identification of interchain or intrachain recombination peaks. Thin films were deposited on glass substrates by drop casting, spin casting, and resonant-infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) and were compared to the photoluminescence of the polymer in solution. The photoluminescence measurements reported in this article demonstrate that samples deposited by evaporative RIR-MAPLE have an internal morphology similar to that of MEH-CN-PPV in solution, leading to an enhancement of intrachain transitions in the conjugated polymer.

Room-temperature, intraband, infrared absorption in CdSe∕poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-(1-cyanovinylene)phenylene] nanocomposites drop cast on GaAs

We have previously demonstrated a method for achieving room-temperature, intraband, midinfrared (3–5μm) absorption in CdSe colloidal quantum dot∕poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4- phenylenevinylene] conducting polymer nanocomposites deposited on GaAs substrates. In this work, theoretical and experimental evidence are presented to further verify that the observed infrared sensitivity is due to intraband transitions.

Resonant Infrared Matrix‐Assisted Pulsed Laser Evaporation Of Inorganic Nanoparticles And Organic/Inorganic Hybrid Nanocomposites

In this research, resonant infrared matrix‐assisted pulsed laser evaporation (RIR‐MAPLE) has been used to deposit different classes of inorganic nanoparticles, including bare, un‐encapsulated ZnO and Au nanoparticles, as well as ligand‐encapsulated CdSe colloidal quantum dots (CQDs). RIR‐MAPLE has been used for thin‐film deposition of different organic/inorganic hybrid nanocomposites using some of these inorganic nanoparticles, including CdSe CQD‐poly[2‐methoxy‐5‐(2’‐ethylhexyloxy)‐1,4‐(1‐cyanovinylene)phenylene] (MEH‐CN‐PPV) nanocomposites and Au nanoparticle‐poly(methyl methacrylate) (PMMA) nanocomposites. The unique contribution of this research is that a technique is demonstrated for the deposition of organic‐based thin‐films requiring solvents with bond energies that do not have to be resonant with the laser energy. By creating an emulsion of solvent and ice in the target, RIR‐MAPLE using a 2.94 μm laser can deposit most material systems because the hydroxyl bonds in the ice component of the emulsion...

RIR-MAPLE deposition of conjugated polymers and hybrid nanocomposites for application to optoelectronic devices

Resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) is a variation of pulsed laser deposition that is useful for organic-based thin films because it reduces material degradation by selective absorption of infrared radiation in the host matrix. A unique emulsion-based RIR-MAPLE approach has been developed that reduces substrate exposure to solvents and provides controlled and repeatable organic thin film deposition. In order to establish emulsion-based RIR-MAPLE as a preferred deposition technique for conjugated polymer or hybrid nanocomposite optoelectronic devices, studies have been conducted to demonstrate the value added by the approach in comparison to traditional solution-based deposition techniques, and this work will be reviewed. The control of hybrid nanocomposite thin film deposition, and the photoconductivity in such materials deposited using emulsion-based RIR-MAPLE, will also be reviewed. The overall result of these studies is the demonstration of emulsion-based RIR-MAPLE as a viable option for the fabrication of conjugated polymer and hybrid nanocomposite optoelectronic devices that could yield improved device performance.

Fourier Transform Infrared Absorbance and Photoluminescence Spectroscopy Studies of CdSe Colloidal Quantum Dot/Conducting Polymer Nanocomposites for Application to Infrared Photodetectors

In this paper we investigate the optical properties of four CdSe colloidal quantum dot/conducting polymer nanocomposites deposited on GaAs substrates using photoluminescence and Fourier transform infrared spectroscopy absorbance. The purpose of this investigation is to find an appropriate electron-conducting polymer for use in a photoconductor that utilizes intraband transitions in the conduction band to detect mid- to long-wave-infrared radiation. As a feasibility demonstration, we fabricate a two-terminal photoconductor and characterize its dark current and spectral responsivity (at 125 K), demonstrating intraband peaks at 0.465 and 0.527 eV, which correspond to 2.67 and 2.35μm.

THEORETICAL INVESTIGATION OF INTRABAND, INFRARED ABSORBANCE IN INORGANIC/ORGANIC NANOCOMPOSITE THIN FILMS WITH VARYING COLLOIDAL QUANTUM DOT SURFACE LIGAND MATERIALS

Hybrid nanocomposite thin films, composed of inorganic colloidal quantum dots (CQDs) embedded in a matrix of organic conjugated polymer, have shown promise as a method for room-temperature infrared detection due to the three-dimensional confinement of the CQD and significantly lower dark currents compared to inorganic detectors. However, in order to improve device performance, the excited charges must be efficiently promoted out of the CQD, which is surrounded by an insulating surface ligand. These short, organic molecules, which are required to prevent agglomeration of CQDs in solution, have been shown to inhibit charge transfer into and out of the CQD. In this work, the transfer matrix method is utilized to calculate the quantized energy levels and wavefunctions in the conduction band of the CdSe CQD for a variety of surface ligand materials. These results are used to calculate the absorption coefficient for a size distribution of CQDs and are compared with measured Fourier Transform Infrared absorbance spectra. Finally, the effect of the ligand on the calculated absorption coefficient will be used to optimize the design for an infrared photoconductor.

Doping effect on carrier occupation and transport in InAs/GaAs quantum dot infrared photodetectors: A capacitance-voltage spectroscopy study

Impurity centers induced by dopants in InAs/GaAs quantum-dot systems affect energy level occupation and carrier transport in multi-layer QDIPs. In order to better understand doping effects and to optimize device performance, capacitance-voltage spectra are investigated.