Changes In Photoconductivity Research Articles

PtTe2 photodetectors with negative photoconductivity under different wavelength laser irradiation

Platinum telluride (PtTe2), as a newly emerged type II Dirac semimetal, has attracted much attention. Due to its excellent linear dispersive band structure and ultra-high carrier mobility, PtTe2 show great potential in the new quantum phenomena, topological phase transitions, unconventional superconductivity and other photodetecting fields. In this paper, we report the negative photoelectric effect of photodetectors based on the type II Dirac semimetal PtTe2 film under the laser irradiation of 405 nm, 532 nm and 808 nm. Under the condition of applying 1 V voltage, the R of the PtTe2 device under 405 nm laser irradiation was 20 A/W and the D* was 6.9×109 Jones; the R of the PtTe2 device under 532 nm laser irradiation was 1.3 A/W and the D* was 4.1×108 Jones; the R of the PtTe2 device under 808 nm laser irradiation was 0.6 A/W and the D* was 2.1×108 Jones. Under normal atmospheric pressure and room temperature conditions, we observed a negative photoconductivity phenomenon for the PtTe2 device, where the change in photoconductivity is attributed to the light-induced desorption of O or H2O molecules from the surface of the material. In addition, a single-site scanning imaging system based on a PtTe2 photodetector was designed to test the detection capability of the device. The analysis of the photocurrent effect produced by the PtTe2 device further deepens the understanding of the photoconductivity mechanism in the field of photodetection.

A new trinuclear Cd(II)-based coordination polymer with Salen ligand building blocks: Synthesis, crystallographic aspects, and optimistic photosensitive Schottky barrier diode behaviour

A new X-ray-characterized Cd(II)-based coordination polymer has been synthesized using Salen ligand (H2L1) and dicyanamide ion (dca) [N(CN)2]−, and characterized using elemental analysis, spectroscopy, SEM/EDX, TEM, and powder X-ray diffraction techniques. According to SCXRD, the complex crystallizes in the triclinic space group P-1. The asymmetric unit includes de-protonated ligands [L]2−, Cd(II) centres, and dca ions. The complex binuclear parts are connected by two μ1,5-dca bridges, resulting in the creation of a one-dimensional coordination polymer [(L1Cd)2(μ1,5-dca)2Cd]n. The polymer undergoes supramolecular aggregation via hydrogen bonding, CH···π(dca), and CH···π(arene) interactions. Hirshfeld surfaces (HS) and 2D fingerprint plots have been deployed to visualize the supramolecular interactions. The analysis shows that N…H interactions (18.2%) are more common in the crystal packing than Cd···O/O···H. This complex displays electrical conductivity and photosensitivity properties, making it a promising material for optoelectronic applications. Experimental and DFT studies have confirmed that it is a direct semiconductor with a specific optical band gap (3.54 eV) and is also used to construct a photosensitive Schottky barrier diode (SBD). The DFT calculated optical conductivity was used to analyse how the material's conductivity changes upon illumination. Due to the photon absorption, the material's electrical conductivity and photoconductivity change were concomitantly observed. Finally, the DOS analysis explains the complex's conductivity behaviour in a better logistic way.

Electron-photon interactions in the conditions of dimensional conductivity restrictions in semiconductor single quantum-size particles in interelectrodic nanogap

For quantum-size particles of the InSb, PbS, HgSe, CdSe semiconductors, a model of competition of dimensional quantum restrictions and blocking with a single-electron current and Coulomb restriction, as well as heating electrons with an electric field of the light wave, is proposed. This made it possible to explain the highly multiplicity (up to two orders of order) of a change in photoconductivity observed in conditions of tunnel conductivity, and in the conditions of the confinement --- the absence of inter-zone and inter-level photoconductivity. The resonant current peaks of quantum conductivity observed on the V-I characteristics when irritating with light of any wavelength (in the interval of 0.4-1.2 μm) are reset or shifted towards smaller voltage values. The energy minimum of quanta recorded in this case is estimated as 100 meV. The results can be useful in resolving issues of application in uncooled IR detectors, including single-photon registration. Keywords: Quantum-size particle, dimensional quantification, one-electron current, single-photon process, inter-zone and intra-zone transitions, tunnel conductivity, Coulomb restriction, quantum conductivity.

Электрон-фотонные взаимодействия в условиях размерного ограничения проводимости в полупроводниковых одиночных квантово-размерных частицах в межэлектродном нанозазоре

For quantum-size particles of the InSb, PbS, HgSe, CdSe semiconductors, a model of competition of dimensional quantum restrictions and blocking with a single-electron current and Coulomb restriction, as well as heating electrons with an electric field of the light wave, is proposed. This made it possible to explain the highly multiplicity (up to two orders of order) of a change in photoconductivity observed in conditions of tunnel conductivity, and in the conditions of the сonfinition - the absence of inter-zone and inter-level photoconductivity. The resonant current peaks of quantum conductivity observed on the V-I characteristics when irritating with light of any wavelength (in the interval of 0.4–1.2 µm) are reset or shifted towards smaller voltage values. The energy minimum recorded in this case is estimated as 100 meV. The results can be useful in resolving issues of application in uncooled IR detectors, including single-photon registration.

Ultrafast Optomechanical Terahertz Modulators Based on Stretchable Carbon Nanotube Thin Films

For terahertz wave applications, tunable and rapid modulation is highly required. When studied by means of optical pump–terahertz probe spectroscopy, single-walled carbon nanotube (SWCNT) thin films demonstrated ultrafast carrier recombination lifetimes with a high relative change in the signal under optical excitation, making them promising candidates for high-speed modulators. Here, combination of SWCNT thin films and stretchable substrates facilitated studies of the SWCNT mechanical properties under strain and enabled the development of a new type of an optomechanical modulator. By applying a certain strain to the SWCNT films, the effective sheet conductance and therefore modulation depth can be fine-tuned to optimize the designed modulator. Modulators exhibited a photoconductivity change of approximately 2 times of magnitude under the strain because of the structural modification in the SWCNT network. Stretching was used to control the terahertz signal with a modulation depth of around 100% without strain and 65% at a high strain operation of 40%. The sensitivity of modulators to beam polarization is also shown, which might also come in handy for the design of a stretchable polarizer. Our results give a fundamental grounding for the design of high-sensitivity stretchable devices based on SWCNT films.

Conversion of n-type to p-type conductivity in ZnO by incorporation of Ag and Ag-Li

Elemental doping is an efficient strategy to modulate different properties of semiconductors. Conversion from n-type to p-type and band gap modulation of ZnO are investigated by theoretical and experimental pathways studies. The structural, electronic and optical properties of undoped, Ag and Ag-Li doped ZnO are studied in the framework of density functional theory (DFT). The value of direct band gap is found to be ~ 0.730, 0.440, 0.274, and 0.870 eV for undoped, Ag = 6.25% and 12.5% doped, and Ag= 6.25, Li= 6.25% co-doped ZnO, respectively. Acceptor levels are created at the top of the valence band and above the Fermi level in Ag and Ag-Li doped ZnO which reveals that Ag and Ag-Li are promising dopants for generating p-type ZnO. Importantly, a remarkable change in photoconductivity and optical properties are observed. The surface morphology of the spray deposited Zn100-xAgxO (x = 0.0–20%) and Zn100-x-yAgxLiyO (x = 5, y = 0.0–10%) thin films are changed with Ag and Ag-Li contents. The XRD patterns confirmed the hexagonal structure of all the deposited films. The band gap decreases from 3.27 to 3.08 eV (for Ag doped ZnO) and increases from 3.16 to 3.38 eV for Ag-Li doping ZnO, respectively. The dielectric constants and photoconductivity spectra support the formation of p-type conductivity of Zn100-xAgxO and Zn100-x-yAgxLiyO, which are in good agreement with the available theoretical and experimental reports. Thus, the studies performed in this work help us to understand the Ag and Ag-Li doping mechanism in ZnO; opening up possible directions toward the fabrication of p-type ZnO for advanced electronic and optoelectronic applications.

Graphene/Si-Based Biosensor for Glioblastoma Cancer Cell Detection

In this paper, a glioblastoma cancer cell biosensor is proposed based on mono-layer graphene/(n-type)Si Schottky junction. Human fibroblast cells are used as a control to study the behavior of the sensor in the presence of healthy cells. It is shown that in the presence of two different glioblastoma cancer cell lines (U87 and T98G) and human fibroblast cells, the photoconductivity of this junction drastically changes under illumination with different wavelengths of light (380 nm to 850 nm). The relative change in photoconductivity of the sensor can not only distinguish the two glioblastoma cancer cell lines, but can also easily differentiate these cancer cells from healthy human fibroblast cells. The proposed sensor is cheap and easily fabricated, and can open a new avenue for detection and differentiation of cancer cells.

A Multi-modal Approach to Understanding Degradation of Organic Photovoltaic Materials.

State-of-the-art organic photovoltaic (OPV) materials are composed of complex, chemically diverse polymeric and molecular structures that form highly intricate solid-state interactions, collectively yielding exceptional tunability in performance and aesthetics. These properties are especially attractive for semitransparent power-generating windows or shades in living environments, greenhouses, or other architectural integrations. However, before such a future is realized, a broader and deeper understanding of property stability must be acquired. Stability during operating and environmental conditions is critical, namely, material color steadfastness, optoelectronic performance retention, morphological rigidity, and chemical robustness. To date, no single investigation encompasses all four distinct, yet interconnected, metrics. Here, we present a multimodal strategy that captures a dynamic and interconnected evolution of each property during the course of an accelerated photobleaching experiment. We demonstrate this approach across relevant length scales (from molecular to visual macroscale) using X-ray photoelectron spectroscopy, grazing-incidence X-ray scattering, microwave conductivity, and time-dependent photobleaching spectroscopies for two high-performance semitransparent OPV blends-PDPP4T:PC60BM and PDPP4T:IEICO-4F, with comparisons to the stabilities of the individual components. We present direct evidence that specific molecular acceptor (fullerene vs nonfullerene) designs and the resulting donor-acceptor interactions lead to distinctly different mechanistic routes that ultimately arrive at what is termed "OPV degradation." We directly observe a chemical oxidation of the cyano endcaps of the IEICO-4F that coincides with a morphological change and large loss in photoconductivity while the fullerene acceptor-containing blend demonstrates a significantly greater fraction of oxygen uptake but retains 55% of the photoconductivity. This experimental roadmap provides meaningful guidance for future high-throughput, multimodal studies, benchmarking the sensitivity of the different analytical techniques for assessing stability in printable active layers, independent of complete device architectures.

Tuning the Photoresponse of Nano-Heterojunction: Pressure-Induced Inverse Photoconductance in Functionalized WO3 Nanocuboids.

Inverse photoconductivity (IPC) is a unique photoresponse behavior that exists in few photoconductors in which electrical conductivity decreases with irradiation, and has great potential applications in the development of photonic devices and nonvolatile memories with low power consumption. However, it is still challenging to design and achieve IPC in most materials of interest. In this study, pressure‐driven photoconductivity is investigated in n‐type WO3 nanocuboids functionalized with p‐type CuO nanoparticles under visible illumination and an interesting pressure‐induced IPC accompanying a structural phase transition is found. Native and structural distortion induced oxygen vacancies assist the charge carrier trapping and favor the persistent positive photoconductivity beyond 6.4 GPa. The change in photoconductivity is mainly related to a phase transition and the associated changes in the bandgap, the trapping of charge carriers, the WO6 octahedral distortion, and the electron–hole pair recombination process. A unique reversible transition from positive to inverse photoconductivity is observed during compression and decompression. The origin of the IPC is intimately connected to the depletion of the conduction channels by electron trapping and the chromic property of WO3. This synergistic rationale may afford a simple and powerful method to improve the optomechanical performance of any hybrid material.

Особенности температурных зависимостей фотопроводимости пленок металлоорганического перовскита CH-=SUB=-3-=/SUB=-NH-=SUB=-3-=/SUB=-PbI-=SUB=-3-=/SUB=-

In this work, the effect of temperature on photoconductivity and its spectral dependence for thin films of metal-organic perovskite CH3NH3PbI3 was studied. Measurements carried out in the temperature region below room temperature revealed specific features of photoconductivity variation with temperature in the phase transition region from the tetragonal to orthorhombic structure (140–170 K). Based on the analysis of the effect of temperature on the spectral dependences of photoconductivity in the region of the phase transition mechanisms are proposed that determine the observed change in photoconductivity

Electron spin resonance identification di-carbon-related centers in irradiated silicon

A previously unreported electron spin resonance (ESR) spectrum was found in γ-ray irradiated silicon by the detection of the change in microwave photoconductivity arising from spin-dependent recombination (SDR). In the specially prepared silicon crystals doped by 13C isotope, a well resolved hyperfine structure of SDR-ESR lines due to the interaction between electrons and two equivalent carbon atoms having nuclear spin I = 1/2 was observed. The Si-KU4 spectrum is described by spin Hamiltonian for spin S = 1 and of g and D tensors of orthorhombic symmetry with principal values g1 = 2.008, g2 = 2.002, and g3 =2.007; and D1 = ± 103 MHz, D2 = ∓170 MHz, and D3 = ± 67 MHz where axes 1, 2, and 3 are parallel to the [11¯0], [110], and [001] crystal axes, respectively. The hyperfine splitting arising from 13C nuclei is about 0.35 mT. A possible microstructure of the detect leading to the Si-KU4 spectrum is discussed.

Green light activated hydrogen sensing of nanocrystalline composite ZnO-In2O3 films at room temperature

The possibility of reducing the operating temperature of H2 gas sensor based on ZnO-In2O3 down to room temperature under green illumination is shown. It is found that sensitivity of ZnO-In2O3 composite to H2 nonmonotonically depends on the oxides’ content. The optimal ratio between the components is chosen. The new mechanism of nanocrystalline ZnO-In2O3 sensor sensitivity to H2 under illumination by green light is proposed. The mechanism considers the illumination turns the composite into nonequilibrium state and the photoconductivity change in the H2 atmosphere is linked with alteration of nonequilibrium charge carriers recombination rate.

Temperature and spectral dependence of CH3NH3PbI3 films photoconductivity

Halide perovskites are widely studied due to their potential applications in solar cells. Despite the remarkable success in increasing perovskite solar cell efficiency, the underlying photophysical processes remain unclear. To cover this gap, we studied temperature, spectral, and light intensity dependence of photoconductivity of CH3NH3PbI3 films in the planar contact configuration. We observed non-monotonic behavior of the photoconductivity temperature dependence: a power-law decrease with increasing temperature at the temperatures below 185 K and close to exponential growth above this temperature. Spectral and light intensity dependences of photoconductivity allowed us to postulate that phase transition between tetragonal and orthorhombic structures and a change in the recombination channel are unlikely to be the reasons for abrupt change in photoconductivity behavior. Charge carrier mobility is proposed to be responsible for unusual photoconductivity changes with temperature.

Photo-controllable memristive behavior of graphene/diamond heterojunctions

Graphene/diamond (carbon sp2-sp3) heterojunctions are demonstrated as photo-controllable memristors with photoswitchable multiple resistance states and nonvolatile memory functions. The ratio of conductivity change between the higher and lower resistance states of the junctions was ∼103. The junctions exhibit light wavelength selectivity, and the resistance states can be switched only by blue or violet light irradiation. The mechanism for the change in photoconductivity is considered to be caused by oxidation-reduction of the graphene and/or graphene-diamond (sp2-sp3) interfaces through the movement of oxygen ions by bias with photo-irradiation because they have wavelength selectivity and require air exposure for several days to exhibit memristive behavior. These results indicate that graphene-diamond, carbon sp2-sp3 heterojunctions can be used as photo-controllable devices with both photomemory and photoswitching functions.

Synthesis of In2S3 and Ga2S3 crystals for oxygen sensing and UV photodetection

Crystal growth, characterization, and sensing application of In2S3 and Ga2S3 are demonstrated herein. Single crystals of In2S3 and Ga2S3 were grown by chemical vapor transport method using ICl3 as the transport agent. The as-grown In2S3 crystals essentially show dark red and Ga2S3 displays transparent and light-yellow colored. X-ray diffraction measurements show tetragonal phase of the as-grown In2S3 crystals while Ga2S3 crystallizes in the monoclinic structure. The optical band edges of In2S3 and Ga2S3 were characterized by absorption and thermoreflectance measurements. The optical band gaps of In2S3 and Ga2S3 are 1.935eV and 3.052eV, respectively. The direct gaps of In2S3 and Ga2S3 lie in between red to violet visible region. Photoluminescence (PL) experiments characterize defect and surface-state emissions of the In2S3 and Ga2S3 crystals. The In2S3 can easily form surface oxide in environmental air with an In2O3 growth rate of ∼100nm/day (i.e. for oxygen sensing). The PL intensity of the In2O3 formed on In2S3 may be a sensing index for oxygen content detection. Besides, Ga2S3 may emit a broadened visible white light (near band edge) due to the existence of intrinsic defects inside the crystal. The Ga2S3 crystal also displays a high-sensitivity photoconductivity change illuminated by a 405nm laser. According to the experimental results, In2S3 can be an oxygen sensor with the formation of surface oxide (i.e. In2O3), while Ga2S3 may possess potential capability in fabrication of blue to UV photodetector.

Properties of Al-Doped ZnO Nanorods Synthesized Using Low Temperature Hydrothermal Method

This work describes the growth of 1-D Al-doped ZnO nanorods via low temperature hydrothermal reaction on seeded substrates. The amount of Al doped ZnO nanorods were tuned by using different concentration of aluminum nitrate from 1-20 mM. The optimum 5 mM Al doping produced an arrays of sharp-tip nanorods with average length of ~1.16 μm and average diameter of ~118 nm. I-V characteristic of the Al-doped ZnO nanorods fabricated onto Al electrodes were observed under UV illumination and dark condition. The change in photoconductivity of Al-doped ZnO nanorods under UV light was found two orders of times higher compared to ZnO nanorods. Different concentrations of aluminium doped ZnO nanorods UV sensing showed response to UV light but with different sensing value. 5 mM Al-doped ZnO showed high responsivity of fabricated UV sensing at 3V with 23.56 A/W which was higher compared to other concentrations. This suggested that the responsivity of Al-doped ZnO NRs UV sensing could be controlled to some extent by controlling the percentage of Al-doped.

Effects of Na incorporation and plasma treatment on Bi2S3 ultra-thin layers

As-deposited bismuth sulfide thin films prepared by means of a chemical bath deposition were treated with argon AC plasma. In this paper, we present the results on the physical modifications which were observed when a pre-treatment, containing a solution of 1M sodium hydroxide, was applied to the glass substrates before depositing the bismuth sulfide. The bismuth sulfide thin films were characterized by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy, atomic force microscopy, UV–VIS, and electrical measurements. The XRD analysis demonstrated an enhancement in the crystalline properties, as well as an increment in the crystal size. The energy band gap value was calculated as 1.60eV. Changes in photoconductivity (σp) values were also observed due to the pre-treatment in NaOH. A value of σp=6.2×10−6 (Ωcm)−1 was found for samples grown on substrates without pre-treatment, and a value of σp=0.28 (Ωcm)−1 for samples grown on substrates with pre-treatment. Such σp values are optimal for the improvement of solar cells based on Bi2S3 thin films as absorber material.

The optically induced and bias-voltage-driven magnetoresistive effect in a silicon-based device

The giant change in photoconductivity of a device based on the Fe/SiO2/p-Si structure in magnetic field is reported. As the magnetic field increases to 1 T, the conductivity changes by a factor of more than 25. The optically induced magnetoresistance effect is strongly dependent of the applied magnetic field polarity, as well as of sign and value of a bias voltage across the device. The main mechanism of the magnetic field effect is related to the Lorentz force, which deflects the trajectories of photogenerated carriers, thereby changing their recombination rate. The structural asymmetry of the device leads to the asymmetry of the dependence of recombination on the magnetic field polarity: recombination of carriers deflected in the bulk of semiconductor is relatively slow, while recombination of carriers at the SiO2/p-Si interface is faster. In the latter case, the interface states serve as effective recombination centers. The bias voltage sign specifies the type of carriers, whose trajectories pass near the interface, providing the main contribution to the magnetoresistance effect. The bias voltage controls the electric field accelerating carriers and, thus, affects the hole and electron trajectories. Moreover, when the bias voltage exceeds a certain threshold value, the electron impact ionization regime is implemented. The magnetic field suppresses impact ionization by enhancing recombination, which makes the largest contribution to the magnetoresistance of the device. The investigated device can be used as a prototype of silicon chips controlled simultaneously by optical radiation, magnetic field, and bias voltage.

Observation of dynamics of impurity photoconductivity in n-GaAs caused by electron cooling

Experimental investigation of the time dependence of impurity photoconductivity in n-GaAs is carried out upon pulsed optical excitation. It is shown that a change in the photoconductivity is determined mainly by electron cooling in the first 20 ns after photoexcitation. A theoretical model for describing the dependences under observation is proposed.

Reversible and irreversible effects after oxygen exposure in thick (>1 μm) silicon films deposited by VHF-PECVD on glass substrates investigated by dual beam photoconductivity

Metastability and instability effects due to oxygen exposure in thick intrinsic hydrogenated microcrystalline silicon films deposited by very high frequency plasma enhanced chemical vapour deposition on smooth glass substrates were investigated using temperature-dependent dark conductivity, steady state photoconductivity, and sub-bandgap absorption measurements obtained using the dual beam photoconductivity (DBP) method. No significant changes in dark conductivity and photoconductivity were detected even after long-term air exposure of samples in room ambient as well as after oxygen exposure when samples were characterized in oxygen ambient. However, characterization of the oxygen-exposed state in high vacuum caused an increase in dark conductivity and photoconductivity as well as a significant decrease in the sub-bandgap absorption coefficient spectra in the low energy region in samples with [Formula: see text]. These changes are partially irreversible for samples [Formula: see text] and mostly reversible for compact materials with significant amorphous fraction. No detectable metastable changes occurred in microcrystalline silicon samples with [Formula: see text] as well as in pure amorphous silicon.