N-type CdSe Research Articles

Self-defined dual charge percolation networks for solution-processed multithreshold transistors

This study demonstrates multithreshold engineering of a solution-processed heterojunction electrochemical transistor using a blend of n-type CdSe tetrapod-shaped nanocrystals (TpNCs) and an n-type polymeric organic semiconductor (OSC). The unique geometry of TpNCs enables a broad concentration range of charge percolation, where charge transfer between TpNC and OSC domains determines multiple threshold voltages. The OSC domain’s threshold voltage shifts from 1 to −1 V as TpNC content increases, while the TpNC domain maintains a threshold above 1.5 V. This allows for stable intermediate states, crucial for multivalued logic operations. Charge percolation and photoluminescence studies show selective charge redistribution, shifting the threshold voltages in the polymer networks. Ternary logic gates, including TNOT, TNAND, and TNOR, based on these heterojunction transistors, were also demonstrated, highlighting the potential of this approach for advanced logic applications.

Band engineering for building cascading charge transport channels in ternary CdS-based nanorod array photoanode toward efficient photoelectrochemical H2 generation

A novel CdS/CdSe/Cu2O nanorod array (NRA) photoelectrode with cascading charge transport channels was designed for the first time to remarkedly boot photoelectrochemical (PEC) hydrogen generation under visible light illumination with zero bias. This ternary photoelectrode was constructed by modifying CdS nanorod arrays with n-type CdSe and p-type Cu2O nanoparticles, successively. In this novel photoanode, CdSe and Cu2O with small band gap not merely highly heighten the visible light absorption capacity of CdS, but more importantly integrate with CdS to build cascading charge transport channels at the CdS/CdSe and CdSe/Cu2O interfaces, dramatically facilitating the separation and transport efficiency for photoexcited electron and hole pairs. The CdS/CdSe/Cu2O NRA photoelectrode gains an obvious hydrogen generation rate of 193.2 μmol·h−1, 8.2 times higher than that of bare CdS NRA photoelectrode. The remarkably enhanced PEC hydrogen production for the CdS/CdSe/Cu2O NRA photoanode can be ascribed to effective solar energy utilization and charge carrier separation.

Electrical effect of CdSe layer thickness deposited on p- Si wafer

In this paper, the current-voltage characteristics of n-CdSe/p-Si heterostructures are analyzed using experimental data and electrical junction characteristics. To develop n-CdSe/p-Si heterojunctions, a wide band gap semiconducting layer of n-type CdSe thin film has been grown on a p-type Si (100) substrate at 100oC with different thickness by using the spray pyrolysis technique. The I-V characteristic of n-CdSe\ p-Si heterostructure has been measured in a room in the dark and under illumination (lamp/160 W). Also, the solar cell IV characteristics and efficiency were measured. The characteristic parameters of the structure such as barrier height, ideality factor, and series resistance were determined from the current-voltage measurement.

(Bi,Sb)2 Se3 Alloy Thin Film for Short-Wavelength Infrared Photodetector and TFT Monolithic-Integrated Matrix Imaging.

Short-wavelength infrared photodetectors play a significant role in various fields such as autonomous driving, military security, and biological medicine. However, state-of-the-art short-wavelength infrared photodetectors, such as InGaAs, require high-temperature fabrication and heterogenous integration with complementary metal-oxide-semiconductor (CMOS) readout circuits (ROIC), resulting in a high cost and low imaging resolution. Herein, for the first time, a low-cost, high-performance, high-stable, and thin-film transistor (TFT) ROIC monolithic-integrated (Bi,Sb)2 Se3 alloy thin-film short-wavelength infrared photodetector is reported. The (Bi,Sb)2 Se3 alloy thin-film short-wavelength infrared photodetectors demonstrate a high external quantum efficiency (EQE) of 21.1% (light intensity of 0.76µWcm-2 ) and a fast response time (3.24µs). The highest EQE is about two magnitudes than that of the extrinsic photoconduction of Sb2 Se3 (0.051%). In addition, the unpackaged devices demonstrate high electric and thermal stability (almost no attenuation at 120°C for 312h), showing potential for in-vehicle applications that may experient such a high temperature. Finally, both the (Bi,Sb)2 Se3 alloy thin film and n-type CdSe buffer layer are directly deposited on the TFT ROIC (with a 64×64-pixel array) with a low-temperature process and the material identification and imaging applications are presented. This work is a significant breakthrough in ROIC monolithic-integrated short-wavelength infrared imaging chips.

Radiation intensity dependence of CdSe photothermal and photocarrier radiometry response evidenced by step-sine modulation method

The radiation intensity dependence of photothermal (PT) and photocarrier (PC) signals from n-type CdSe single crystals was investigated by modulated infrared radiometry (MIRR) in the mid-IR range (5–11.3 μm) with superbandgap photoexcitation. The influence of dc temperature increase of the sample was avoided by a new step-sine modulation method that combines the advantages of transient and periodical modulation. With increasing laser intensity I, the amplitude of the PC component shows a sub-linear dependence (|SPC| ∝ I0.5), while the PT one has the expected linear dependence (|SPT| ∝ I). As a result, the transition frequency ft between the two components is shifted to higher frequencies, which is explained in the frame of a simple model. The origin of the observed effects is the decrease of the effective photocarrier lifetime τ ∝ I−0.5 over three laser intensity decades. In contrast, previous studies on nonlinear PC response in semiconductors performed in the near-IR range (0.7–1.8 μm) have found supra-linear |SPC| dependence with exponent between 1 and 2. This difference is attributed to the fact that the near-IR radiometric signal features characteristics of a photoluminescence (PL) signal that are different from those of the mid-IR PC signal, as shown in our previous study [J. Appl. Phys. 119, 125108 (2016)] on the same CdSe samples.

Epitaxial CdSe/PbSe Heterojunction Growth and MWIR Photovoltaic Detector

A novel Epitaxial Cadmium Selenide (CdSe) on Lead Selenide (PbSe) type-II heterojunction photovoltaic detector has been demonstrated by Molecular Beam Epitaxy (MBE) growth of n-type CdSe on p-type PbSe single crystalline film. The use of Reflection High-Energy Electron Diffraction (RHEED) during the nucleation and growth of CdSe indicates high-quality single-phase cubic CdSe. This is a first-time demonstration of single crystalline and single phase CdSe growth on single crystalline PbSe, to the best of our knowledge. The current-voltage characteristic indicates a p-n junction diode with a rectifying factor over 50 at room temperature. The detector structure is characterized by radiometric measurement. A 30 μm × 30 μm pixel achieved a peak responsivity of 0.06 A/W and a specific detectivity (D*) of 6.5 × 108 Jones under a zero bias photovoltaic operation. With decreasing temperature, the optical signal increased by almost an order of magnitude as it approached 230 K (with thermoelectric cooling) while maintaining a similar level of noise, achieving a responsivity of 0.441 A/W and a D* of 4.4 × 109 Jones at 230 K.

Electrical Properties of n-CdSe/Si-p Junction Prepared by Chemical Bath Deposition Technique, from Different Weights of Sodium Selenosulfate and Constant Molarity

A pn-junction was successfully fabricated by depositing n-type CdSe thin films on p-type Si as a substrate using chemical bath deposition technique (CBD) at 70oC. Time of deposition was 6 hours and the preparing solution was changed every 2 hours during the deposition. Sodium Selenosulfate (with different weights) is the source of Se-2 ions, cadmium nitrate is the source of Cd+2 ions. The (I-V) characteristics for the n-CdSe/p-Si junction show it behaves as a Zener diode in reverse bias, with Zener resistance (3 and 27×103)W. SEM also shows spherical-shape particles with difference grain size (3.8 and 19.8) nm.

Studies on Ni/(n)CdSe thin film Schottky barrier junction

Ag doped n-type CdSe thin films of doping concentration up to 5.21 × 1014 cm−3 have been used to fabricate Schottky barrier junction with Ni metal on glass substrates by sequential thermal evaporation. All the junctions of different doping concentrations exhibited rectifying current–voltage (I-V) characteristics with a non-saturating reverse current. From the current density–voltage (J-V) characteristics, the junction parameters such as ideality factor, saturation current density, series resistance, etc., were measured. The junctions were found to possess higher values of ideality factor and series resistance. The Richardson constant evaluated from ln(J0/T2) vs T−1 plot was found to be 80 after heat treatment and the barrier height was around 0.62 eV. While the barrier height obtained from C-V plot was around 0.65 eV. The structure exhibited poor photovoltaic effect characterized by a fill factor not>0.36. The diode quality as well as photovoltaic performance of the junctions was improved after heat treatment in vacuum at 373 K for 1 hr.

Performance of the Yb/n-CdSe/C Tunneling Barriers

In this article, the design and performance of the CdSe which are deposited onto thin films of Yb metal is reported and discussed. The thin films of CdSe which are deposited by the physical vapor deposition technique are observed to exhibit slightly deformed hexagonal polycrystalline nature with excess amount of Cd as confirmed by the X-ray, energy dispersive X-ray spectroscopy and scanning electron microscopy techniques. The n-type CdSe is also found to form a Schottky barrier of tunneling type when sandwiched between Yb and carbon. The quantum mechanical tunneling mechanism in this device which was tested and modeled in the frequency domain of 10–150 MHz is found to exhibit average intersite separations of ∼5 nm. The tunneling device exhibited a widening in the depletion region associated with significantly large capacitance tunability in the studied frequency domain. On the other hand, as an optoelectronic device, the Yb/n-CdSe/C Schottky diode exhibited a responsivity of ∼0.10 A/W, photosensitivity of 6.5 × 104 and external quantum efficiency of 54% when biased with 1.0 V and exposed to laser light of wavelength of 406 nm.

Efficient decontamination of textile industry wastewater using a photochemically stable n-n type CdSe/Ag3PO4 heterostructured nanohybrid containing metallic Ag as a mediator.

To address the environmental hazard posed by the discharge of textile industry wastewater, we combined n-type (CdSe) and n-type (Ag3PO4) visible-light-responsive semiconductors to produce a photochemically stable n-n type heterostructured nanohybrid comprising metallic Ag (CdSe/Ag/Ag3PO4, CAA) and employed this material as a catalyst for the decomposition of phenol and rhodamine B (RhB). The physicochemical properties of CAA and reference photocatalysts were investigated using instrumental techniques. Compared to individual Ag3PO4 and CdSe, CAA showed an elevated photocatalytic decomposition efficiency for both target pollutants, which was mainly attributed to increased charge separation efficiency and explained by the operation of the Z-scheme reaction mechanism. Moreover, we probed the effects of initial pollutant concentration, AgNO3:NaH2PO4 molar ratio, and the CdSe:Ag3PO4 mass ratio of CAA on catalytic performance. Recycling tests revealed the high photochemical stability of CAA, which was ascribed to the prevention of Ag3PO4 photoreduction by electrons. Finally, a Z-scheme mechanism with vectorial charge transfer suggested for the visible-light-driven decomposition of pollutants over CAA nanohybrids was systematically discussed based on the results of scavenger tests and photoluminescence emission spectra analysis, and an explanation of the role of metallic Ag as a charge mediator was provided.

Gold and ytterbium interfacing effects on the properties of the CdSe/Yb/CdSe nanosandwiched structures

Owing to the performance of the CdSe as an optoelectronic material used for the production of quantum dots, photosensors and wave traps we here, in this article, report the enhancements in structural and electrical properties that arises from the nanosandwiching of a 40 nm thick Yb film between two films of CdSe (CYbC-40). The CdSe films which were deposited onto glass, Yb and Au substrates are characterized by X-ray diffraction, temperature dependent electrical conductivity and impedance spectroscopy measurements in the frequency range of 10–1800 MHz. The analysis of the XRD patterns have shown that the glass/CdSe/Yb/CdSe films exhibit larger grain size and lower strain, defect density and lower stacking faults compared to the not sandwiched CdSe. In addition, it was observed that the Yb shifts the donor states of the n-type CdSe from 0.44 to 0.29 eV leading to a modification in the built in voltage of the material. On the other hand, the design of the energy band diagram has shown the ability of the formation of the Au/CYbC-40/Yb as Schottky (SB) and the Au/CYbC-40/Au as back to back Schottky barriers (BBSB). While the SB device show low band pass filter characteristics, the BBSB device performed as band stop filters. The BBSB device exhibited negative capacitance effects with filtering features that reveal a return loss of 42 dB at ∼1440 MHz.

Hybrid Black Phosphorus/Zero-Dimensional Quantum Dot Phototransistors: Tunable Photodoping and Enhanced Photoresponsivity.

Recently, black phosphorus (BP) with direct band gap exhibited excellent potential for optoelectronic applications because of its high charge carrier mobility and low dark current as well as the variable band gap of 0.3-1.5 eV depending on the number of layers. However, few-layer BP-based phototransistors (photo-FETs) have been limited in sensitivity and wavelength selectivity. To overcome the drawback of these photo-FETs, we studied hybrid photo-FETs combined with the novel properties of the two materials between the channel and sensitizer layers. By combining a strong absorbance of a quantum dot (QD) layer and a two-dimensional layer material with high carrier mobility, the hybrid photo-FETs are expected to produce high-performance photodetectors that can effectively control the responsivity, detectivity, and response time. In this study, we demonstrate that the photogenerated carriers formed from QD sensitizer layers migrate to the BP transport layer with high charge mobility and not only improve the photodetector performance but also enhance the photodoping effect of the BP transport layer with an ambipolar characteristic by electrons transferred from n-type CdSe QDs or holes injected from p-type PbS QDs. The responsivity and detectivity of hybrid BP/0D photo-FETs exhibit 1.16 × 109 A W-1 and 7.53 × 1016 Jones for the BP/CdSe QD photo-FET and 5.36 × 108 A W-1 and 1.89 × 1016 Jones for the BP/PbS QD photo-FET, respectively. The photocurrent rise (τrise) and decay (τdecay) times were τrise = 0.406 s and τdecay = 0.815 s for BP/CdSe QD photo-FET and τrise = 0.576 s and τdecay = 0.773 s for BP/PbS QD photo-FET, respectively.

Periodic voltammetry as a successful technique for synthesizing CdSe semiconductor films for photo-electrochemical application

Polycrystalline n-type CdSe thin films were deposited from aqueous bath at room temperature by periodic voltammetry for use in photo-electrochemical (PEC) solar cells. The influence of periodic scans on the functional properties of the chalcogenide films was thoroughly investigated through electron microscopy (SEM), X-ray diffraction (XRD), UV–visible spectrometry, and electrochemical techniques. The film thickness increases with the number of periodic scans, and the spectral measurements identify the optical band gap in the range 1.55–1.72 eV. The CdSe films show typical X-ray diffraction patterns of hexagonal crystal structure with particle size 20–30 nm, uniformly distributed throughout the matrix. PEC characteristics including photo-current density, conversion efficiency (η), fill factor (FF), and durability of the films were evaluated by the help of respective electrochemical techniques. The voltammetric cycle number was optimized which enable to identify the best CdSe film exhibiting photo-conversion efficiency of ~1% with minimum photocurrent decay.

Spectroelectrochemical Measurement of Surface Electrostatic Contributions to Colloidal CdSe Nanocrystal Redox Potentials

Understanding and controlling the redox properties of colloidal semiconductor nanocrystals is critical for application of this class of materials in many proposed technologies. Here, we use spectroelectrochemical potentiometry to analyze the redox potentials of free-standing colloidal n-type CdSe nanocrystals. We show that both the redox potentials and the maximum number of conduction-band electrons that can be accumulated through photodoping are strongly affected by the nanocrystal’s surface stoichiometry, varying reproducibly by over 400 meV with changes in relative Cd2+:Se2– surface concentration. The data suggest that Se2– enrichment generates electric dipoles at the nanocrystal surfaces that shift the CdSe nanocrystal band-edge potentials to more negative values, and these generated dipoles are largely eliminated upon Cd2+ binding. These results demonstrate the importance of nanocrystal surface stoichiometry in applications involving tuned nanocrystal redox potentials, band-edge alignment, or electro...

Current Enhancement of CdTe-Based Solar Cells

We report on the realization of CdTe solar cell photocurrent enhancement using an n-type CdSe heterojunction partner sputtered on commercial SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> /SnO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> :F coated soda-lime glass substrates. With high-temperature close-space sublimation CdTe deposition followed by CdCl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> activation, this thin-film stack allows for substantial interdiffusion at the CdSe/CdTe interface facilitating a CdSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Te <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</sub> alloy formation. The bowing effect causes a reduced optical bandgap of the alloyed absorber layer and, therefore, leads to current enhancement in the long-wavelength region and a decrease in open-circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> ). To overcome the V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> loss and maintain a high short-circuit current (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> ), the CdTe cell configuration has been modified using combined CdS:O/CdSe window layers. The new device structure has demonstrated enhanced collection from both short-and long-wavelength regions as well as a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> improvement. With an optimized synthesis process, a small-area cell using CdS:O/CdSe window layer showed an efficiency of 15.2% with a V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> of 831 mV, a J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> of 26.3 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and a fill factor of 69.5%, measured under an AM1.5 illumination without antireflection coating. The results provide new directions for further improvement of CdTe-based solar cells.

Construction of crossed heterojunctions from p-ZnTe and n-CdSe nanoribbons and their photoresponse properties

Sb-doped p-type ZnTe nanoribbons (NRs) and Ga-doped n-type CdSe NRs were synthesized via a co-thermal evaporation method in a horizontal tube furnace, respectively. Crossbar heterojunction diode (HD) devices were constructed from p-ZnTe:Sb NRs and n-CdSe NRs by a convenient route. The p-ZnTe/n-CdSe NR HD device exhibits a significant rectification characteristic with a rectification ratio up to 103 within ±5 V and a low turn-on voltage of 2.6 V. Photoresponse analysis reveals that such HD devices were highly sensitive to light illumination with excellent stability, reproducibility and fast response speeds of 37/118 μs at reverse bias voltage. It is expected that such HD devices will have great potential applications in electronic and optoelectronic devices in the future.

Minority carrier recombination lifetimes in n-type CdMgSe mixed crystals measured by means of the photothermal infrared radiometry

AbstractMinority recombination lifetimes of n-type CdMgSe mixed crystals were estimated by using infrared photothermal radiometry (PTR) amplitude and phase frequency spectra. The results obtained by the PTR method indicate that the lifetimes of optically generated carriers in CdSe and CdxMg1−xSe crystals are about 0.1 μs. The diffusion length of minority carrier in n-type CdSe single crystal was found to be 4.42 μm and it is in a good agreement with the literature value. It was found that with the increasing thermal-to-plasma component coefficient A the carrier concentration increases as expected from PTR theory.

Size Dependence of Negative Trion Auger Recombination in Photodoped CdSe Nanocrystals

We report a systematic investigation of the size dependence of negative trion (T(-)) Auger recombination rates in free-standing colloidal CdSe nanocrystals. Colloidal n-type CdSe nanocrystals of various radii have been prepared photochemically, and their trion decay dynamics have been measured using time-resolved photoluminescence spectroscopy. Trion Auger time constants spanning 3 orders of magnitude are observed, ranging from 57 ps (radius R = 1.4 nm) to 2.2 ns (R = 3.2 nm). The data reveal a substantially stronger size dependence than found for bi- or multiexciton Auger recombination in CdSe or other semiconductor nanocrystals, scaling in proportion to R(4.3).

Photochemical Electronic Doping of Colloidal CdSe Nanocrystals

A method for electronic doping of colloidal CdSe nanocrystals (NCs) is reported. Anaerobic photoexcitation of CdSe NCs in the presence of a borohydride hole quencher, Li[Et3BH], yields colloidal n-type CdSe NCs possessing extra conduction-band electrons compensated by cations deposited by the hydride hole quencher. The photodoped NCs possess excellent optical quality and display the key spectroscopic signatures associated with NC n-doping, including a bleach at the absorption edge, appearance of a new IR absorption band, and Auger quenching of the excitonic photoluminescence. Although stable under anaerobic conditions, these spectroscopic changes are all reversed completely upon exposure of the n-doped NCs to air. Chemical titration of the added electrons confirms previous correlations between absorption bleach and electron accumulation and provides a means of quantifying the extent of electron trapping in some NCs. The generality of this photodoping method is demonstrated by initial results on colloidal CdE (E = S, Te) NCs as well as on CdSe quantum dot films.

Nanoscale assembly into extended and continuous structures and hybrid materials

Harnessing synthetic power to create novel materials with unique assembly properties is of fundamental interest for pushing the limits of molecular and macromolecular assembly, while further holding the potential for improving electronic and medical device performance. Controlling the assembly of aromatic molecules into nanostructures provides routes towards continuous and extended structures with performance that is improved markedly over that of their individual molecular components, opening possibilities for the formation of composites that have tailored interactions with other materials and thus improved applications. This review covers recent advances in the synthesis of conjugated materials, their controlled assembly into nanoscale architectures and implementation into devices. Specifically, we discuss the solution-based assembly of polythiophene into nanowire fibrils, the formation of columnar stacks of hexabenzocoronene liquid crystals and the preparation of functionalized solution processible graphene for nanocomposite formation. When polymers are mentioned, it is ‘plastics’ that typically spring to mind. Yet the assembly of conjugated polymers or aromatic molecules into hierarchical and hybrid structures also brings about materials that are attractive for electronic and medical devices. Emily Pentzer and Todd Emrick from the University of Massachusetts review recent advances into the preparation of well-defined molecular building blocks, their assembly into complex architectures, and the properties emerging from these arrangements. This approach is illustrated by the variety of assemblies featuring conjugated polythiophene chains, such as nanowires, fibrils that can further aggregate into ribbons, or composites with quantum dots. These higher-order structures display a variety of electronic properties, useful in semiconductor and photovoltaic applications among others. Another example of the potential of this approach is the construction of carbon nanotubes or graphene flakes from aromatic molecules. Pentzer and Emrick also highlight the remaining challenges that relate to controlling the structures obtained, their stability, and their integration into devices. Poly(3-hexylthiophene) nanowires, microns in length and nanometers in diameter, are stably suspended in solution, and crystallize by adjusting solvent strength. Cocrystalization of P3HT with P3HT-covered CdSe nanorods gave hybrid nanostructures in which the p-type P3HT fibrils were flanked by the n-type CdSe nanorods. The close proximity and arrangement of the two materials provides direct, continuous pathways suitable for charge transport in photovoltaic devices.