Negative Photoconductivity Research Articles

Charge transfer in the fullerene C60-few layer graphene system and the existence of negative photoconductivity

The electrical conductance of few layer graphene during thermal evaporation of fullerene C60 as well as the conductance under illumination of this bilayer junction is reported. We obtained few layer graphene films by chemical vapor deposition technique, and then C60 was thermally evaporated on these films. We found an increase in conductance of few layer graphene during the thermal evaporation of C60 and a kind of negative photoconductivity when the bilayer junction was under illumination. We observed that at low light intensities few layer graphene is p-type doped, and we propose that its behavior changes to n-type for high light intensities.

SbSI Single Nanowires as Humidity Sensors

Humidity sensing using oneand two-dimensional nanostructures is investigated in many research centers around the world. The sensors constructed from nanomaterials have received great attention because of their enormous surface-to-volume ratios. It makes their electrical properties extremely sensitive to species adsorbed on surfaces. Antimony sulfoiodide (SbSI) is a semiconducting ferroelectric. Recently, the influence of humidity on impedance spectra [1] and photoconductivity transient characteristics [2] of SbSI gel, made up of large quantity of nanowires, has been studied. In order to obtain simpler image of the observed phenomena, the experiments with single SbSI nanowires are analyzed in this paper. Measurements of photocurrent transient characteristics (Fig. 1a) have been performed for different relative humidities in ferroelectric phase (T=280 K) using argon laser (λ=488 nm). Figure 1b presents the influence of water vapor on DC conductivity and photoconductivity of SbSI single nanowires. While negative photoconductivity is observed for SbSI gel [2], only the positive effect occurs for single SbSI nanowires, even for high RH.

Photo-activated oxygen sensitivity of graphene at room temperature

Photo-induced changes in the electrical conductivity and the sensitivity to oxygen gas of graphene sheets grown by chemical vapor deposition and transferred onto Al2O3 and SiO2 thin film substrates were studied at ambient conditions. The pristine graphene sensors were initially completely insensitive to oxygen gas at room temperature but showed significant (up to 100%) response when illuminated with weak ultraviolet (300 nm or 365 nm) light. Oxygen response was governed by Langmuir law and its activation was insensitive to humidity. The mechanism of sensitization is analyzed together with other photo-induced effects—negative persistent photo-conduction and photo-induced hysteresis of field effect transistor characteristics. While the reduction of conductivity in air is persistent effect, the oxygen sensitization and enlargement of hysteresis take place only under the direct influence of light. It is concluded that the charge traps with differently adsorbed oxygen and water are involved in these phenomena.

Competing Ultrafast Energy Relaxation Pathways in Photoexcited Graphene

For most optoelectronic applications of graphene, a thorough understanding of the processes that govern energy relaxation of photoexcited carriers is essential. The ultrafast energy relaxation in graphene occurs through two competing pathways: carrier-carrier scattering, creating an elevated carrier temperature, and optical phonon emission. At present, it is not clear what determines the dominating relaxation pathway. Here we reach a unifying picture of the ultrafast energy relaxation by investigating the terahertz photoconductivity, while varying the Fermi energy, photon energy and fluence over a wide range. We find that sufficiently low fluence (≲4 μJ/cm(2)) in conjunction with sufficiently high Fermi energy (≳0.1 eV) gives rise to energy relaxation that is dominated by carrier-carrier scattering, which leads to efficient carrier heating. Upon increasing the fluence or decreasing the Fermi energy, the carrier heating efficiency decreases, presumably due to energy relaxation that becomes increasingly dominated by phonon emission. Carrier heating through carrier-carrier scattering accounts for the negative photoconductivity for doped graphene observed at terahertz frequencies. We present a simple model that reproduces the data for a wide range of Fermi levels and excitation energies and allows us to qualitatively assess how the branching ratio between the two distinct relaxation pathways depends on excitation fluence and Fermi energy.

Semiconducting-to-metallic photoconductivity crossover and temperature-dependent Drude weight in graphene.

We investigate the transient photoconductivity of graphene at various gate-tuned carrier densities by optical-pump terahertz-probe spectroscopy. We demonstrate that graphene exhibits semiconducting positive photoconductivity near zero carrier density, which crosses over to metallic negative photoconductivity at high carrier density. These observations can be accounted for by the interplay between photoinduced changes of both the Drude weight and carrier scattering rate. Our findings provide a complete picture to explain the opposite photoconductivity behavior reported in (undoped) graphene grown epitaxially and (doped) graphene grown by chemical vapor deposition. Notably, we observe nonmonotonic fluence dependence of the photoconductivity at low carrier density. This behavior reveals the nonmonotonic temperature dependence of the Drude weight in graphene, a unique property of two-dimensional massless Dirac fermions.

Studies on the Growth and Characterization of Organic L-Arginine Semicarbazone Dihydrate NLO Single Crystal

This paper reports an organic crystal of a New Nonlinear Optical (NLO) material, L-Arginine Semicarbazone dihydrate (abbreviated as LASC). The crystals were grown by slow evaporation from an aqueous solution and crystallize in monoclinic form which is confirmed by single crystal x-ray diffraction analysis. The molecular weight of the grown crystal was estimated by mass spectrometry. The FTIR spectra con-firm the presence of the functional groups in the crystal while The UV-vis-NIR spectrum of grown crystal LASC reveals its transparent nature. The optical absorption in the grown crystal is almost negligible in the UV and visible spectral regions with the lower cut off wavelength at around 350 nm. Minimum amount of optical absorption suggests that the LASC crystal is highly transparent in the UV-visible spectral region and is useful for nonlinear optical applications. The second harmonic generation is confirmed by the emission of green light and its efficiency is found to be 0.51 times that of KDP crystal. The photoconductivity study shows that the grown crystal exhibit negative photoconductivity. Primary studies show that the grown crystals are appropriate for optoelectronics applications and Second Harmonic Generations.

Synthesis, Growth, and Characterization of Bisglycine Hydrobromide Single Crystal

Single crystals of BGHB were grown by slow evaporation technique. The unit cell dimensions and space group of the grown crystals were confirmed by single crystal X-ray diffraction. The modes of vibration of the molecules and the presence of functional groups were identified using FTIR technique. The microhardness study shows that the Vickers hardness number of the crystal increases with the increase in applied load. The optical properties of the crystals were determined using UV-Visible spectroscopy. The thermal properties of the grown crystal were also determined. The refractive index was determined as 1.396 using Brewster’s angle method. The emission of green light on passing the Nd: YAG laser light confirmed the second harmonic generation property of the crystals and the SHG efficiency of the crystals was found to be higher than that of KDP. The dielectric constant and dielectric loss measurements were carried out for different temperatures and frequencies. The ac conductivity study of the crystals was also discussed. The photoconductivity studies confirm that the grown crystal has negative photoconductivity nature. The etching studies were carried out to study the formation of etch pits.

Negative photoconductivity in films of alloys of II–VI compounds

Various negative photoelectric effects in films of alloys of II–VI compounds deposited from a solution are studied depending on the deposition mode and heat treatment. A combined electronic-molecular mechanism of the negative photocapacitance effect, which is for the first time found in Cd1 − xZnxS and CdS1 − xSex films, and negative slowly relaxing photoelectric effects is established. The latter are caused by the transition of electrons arranged in a nanoscale surface layer from shallow energy levels of attachment centers to deeper levels with lower polarizability and by the presence of nanoscale clusters playing the role of a “reservoir” for minority carriers in these materials. A model, which makes it possible to interpret the main regularities of negative photoconductivity in Cd1 − xZnxS and CdS1 − xSex films deposited from a solution, is suggested. It is established that the negative residual photoconductivity is explained on the basis of a double barrier profile, while the negative differential photoconductivity is explained by the presence of nanoscalel electric domains.

Photocurrent response of carbon nanotube–metal heterojunctions in the terahertz range

We investigate the optoelectronic properties of a carbon nanotube (CNT)-metal heterostructure in the terahertz range. On the basis of terahertz time-domain spectroscopy characterization of a double-walled CNT (DWNT) film, we present and analyze the photocurrent measurement for a DWNT-nickel heterojunction illuminated by continuous-wave terahertz radiation. A significant current across the junction directly induced by terahertz excitation is observed and a negative photoconductivity behavior is found to occur in the device. The photocurrent shows a linear response to the bias voltage and the illumination power within the examined range. These phenomena support the feasibility of using CNT-metal heterojunctions as novel terahertz detectors.

Transient characteristics and negative photoconductivity of SbSI humidity sensor

This paper presents the influence of water vapor on the photoconductivity of antimony sulfoiodide (SbSI) nanowires obtained sonochemically. It is done to explore the applicability of SbSI photoconductor as a humidity sensor. Character of the response of DC photoconductivity current on the switching on and switching off illumination of SbSI gel depends on relative humidity (RH) of nitrogen environment. While positive photoconductivity is observed for low RH, the negative effect occurs for high RH. The least square fitting of the transient characteristics of photoconductivity allowed to present equivalent photoelectric model of SbSI gel. The changes of the parameters of the model with increasing humidity and illumination intensity are presented. Mechanism of light-induced desorption of H2O from SbSI nanowires’ surface is discussed.

Structural, optical and photoconductivity study of ZnS nanoparticles synthesized by a low temperature solid state reaction method

In the present work, ZnS nanoparticles have been synthesized by a simple solid state reaction method at different temperatures. X-ray diffraction patterns show that the synthesized ZnS nanoparticles have a zinc blende structure of ZnS. SEM micrographs show formation of spherical nanoparticles of ZnS. UV–vis spectra show blue-shifting in an absorption edge corresponding to all the synthesized ZnS nanoparticles as compared to their bulk counterparts. FTIR study is performed to determine the absorbance and nature of bonds present in synthesized samples. Photoconductivity properties have been investigated by varying voltage and as a function of time. Photocurrent varies linearly for all the synthesized samples. Rise and decay exhibit negative photoconductivity in samples synthesized at lower temperatures.

Photoelectric Properties of Photoconducting Composites Based on Non-Photoconducting Polymers Doped by Triarylmethane and Xanthene Dyes

The films of polymeric composites based on non-photoconducting polyvinyl alcohol and polystyrene doped by various concentration of xanthene and triarylmethane type dyes have been prepared. Their photophysical, electrical, and spectral properties have been investigated. It was shown, that all studied composite films exhibit photovoltaic properties and anomalous negative photoconductivity effect. The diffusion nature of observed photovoltaic effect was established. The features of photovoltaic effect and photoconductivity, as well as the possible negative photoconductivity mechanism are discussed.

Third order nonlinear optical, luminescence and electrical properties of bis glycine hydrobromide single crystals

Optical quality bis glycine hydrobromide (BGHB) single crystal was grown by slow evaporation technique. The third order nonlinear refractive index and nonlinear absorption coefficient of the grown crystal were measured by Z-scan studies. The third order nonlinear susceptibility was found to be 9.612×10−4esu which is fairly higher than the other glycine compounds. The Photoluminescence spectra reveal the emission bands for BGHB crystals. The band gap energy was calculated to be 3.1eV. The Photoconductivity studies were employed to determine the dependence of photocurrent on the applied electric field. Negative photoconductivity was exhibited by the sample. The d.c. conductivity of the grown crystal was measured by the complex impedance analysis wherein the obtained plot in the form of semicircle finds application in Debye relaxation for materials having large dc conductivity.

Negative photoconductance in a biased multiquantum well with filter barriers

In this paper the photon-assisted electron motion in a multiquantum well (MQW) semiconductor heterostructure in the presence of an electric field is investigated. The time-dependent Schrodinger equation is solved by using the split-operator technique to determine the photocurrent generated by the electron movement through the biased MQW system. An analysis of the energy shifts in the photocurrent spectra reveals interesting features coming from the contributions of localized and extended states on the MQW system. The photocurrent signal is found to increase for certain values of electric field, leading to the analogue of the negative-conductance in resonant tunneling diodes. The origin of this enhancement is traced to the mixing of localized states in the QWs with those in the continuum. This mixing appears as anticrossings between the localized and extended states and the enhanced photocurrent can be related to the dynamically induced Landau-Zener-Stuckelberg-Majorana transition between two levels at the anticrossing.

Deep level states and negative photoconductivity in n-ZnO/p-Si hetero-junction diodes

Negative photoconductivity (NPC) was observed in n-ZnO/p-Si heterojunction diode grown by ultra-high vacuum sputtering method under nitrogen ambient. Under the illumination of ultra-violet light, positive photoconductivity was observed at low bias voltages, whereas NPC was observed at high bias voltages. The defect states in the ZnO layers grown on Si were analyzed by photoluminescence and deep level transient spectroscopy measurements. Two deep levels were measured at Ec-0.51 eV and Ec-0.54 eV, which might be originated from oxygen vacancy and nitrogen atom related defects, respectively. Based on the simulation of band diagram, the defect states were located below Fermi level at zero bias voltage. However, as increasing the bias voltages, NPC was observed due to the increase of empty defect states. This analysis allowed us to consider the possibility that the NPC phenomenon in n-ZnO/p-Si heterojunction diode is originated dominantly from the defect states as a carrier recombination center in ZnO layer.

Effect of Cu concentration on the photoconductivity properties of ZnS nanoparticles synthesized by co-precipitation method

In the present work, solution based co-precipitation method has been used to prepare Cu-doped ZnS nanoparticles and characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), UV–vis absorption spectroscopy and photoconductivity (PC). XRD analysis shows the cubical zinc blende structure while for higher doping concentration extra peaks of CuS have been observed. The estimated crystallite sizes of Cu-doped ZnS lies in the range of 2–4nm. SEM micrographs show spherical shape of nanoparticles. UV–vis absorption study shows blue shift of absorption shoulder for un-doped and Cu-doped ZnS NPs as compared to the bulk counterpart. In photoconductivity study, the enhancement of photosensitivity has been observed in 4% Cu-doped sample while the maximum current is found in 1% Cu-doped sample. The substitution of Cu is found to affect the photoconductivity properties owing to the CuS phase. The variation of darkcurrent and photocurrent with applied voltages exhibits super-linear nature (r>1). Negative photoconductivity (NPC) is observed in all the samples in which the photocurrent decreases during steady illumination.

Photocurrent saturation and negative differential photoconductivity in Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M heterojunctions

A mechanism behind the saturation of the photocurrent and occurrence of negative differential photoconductivity in Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M heterojunctions is found. Mn4Si7-Si〈Mn〉-Mn4Si7 and Mn4Si7-Si〈Mn〉-M structures are studied with a model of back-to-back diodes. Photocurrent-voltage characteristics are taken at high constant and pulsed applied biases. It is found that the nonlinearity of the photocurrent-voltage characteristics and photoconductivity kinetics are due to the quenching of photoconductivity by Joule self-heating.

Crossover of persistent photoconductivity in a phase-separated La0.325Pr0.3Ca0.375MnO3 thin film

A crossover from positive to negative persistent photoconductivity (PPC) has been observed in the thermal hysteresis region of a La0.325Pr0.3Ca0.375MnO3 thin film. In the cooling process, the resistance shows downward relaxation in darkness. Light illumination induces a resistance drop and positive PPC is observed. However, upon warming, the resistance exhibits upward relaxation without illumination. Moreover, the response of the film to light illumination shows distinct behavior. The resistance decreases to a lower value when the illumination is switched on and then shows upward relaxation during illumination. It recovers to a higher value when the illumination is stopped, which characterizes negative PPC. The PPC ratio is −30.2% at 125 K with a light intensity of 3.06 mW mm−2. These results are discussed based on the phase competition and phase stability switch between ferromagnetic metal and charge/orbital-ordered insulator states. The results may be important for practical applications in photoelectric devices.

Temperature sensitive photoconductivity observed in InN layers

Photoconductivity has been systematically studied in unintentionally doped n-type InN film with super-bandgap excitation (1.53 eV) at temperatures varying in the range of 100–300 K. A negative photoconductivity is observed at room temperature, whereas it gradually changes to be positive with decreasing temperature. Transition temperature from negative to positive photoconductivity is found to be greatly related to the residual electron concentration as the higher the electron concentration, the lower the transition temperature. An energy band model including a donor state with large lattice relaxation as well as a recombination center is proposed, which explains the experimental observation well.

Structural, spectral, thermal, dielectric, mechanical and optical properties of L-threonine cadmium acetate dihydrate a semiorganic nonlinear optical crystals

Semiorganic nonlinear optical crystal of L-threonine cadmium acetate dihydrate (LTCA) has been grown from aqueous solution by slow evaporation technique at room temperature. The grown crystal was subjected to single-crystal X-ray diffraction and powder X-ray diffraction analysis to find the lattice parameter and the structure of the crystal. The vibrational frequencies of various functional groups in LTCA were confirmed by Fourier transform infrared spectroscopy (FTIR) analysis. The UV–vis–NIR spectrum of the crystal shows the lower cut-off wavelength at 289nm. The fluorescence spectrum confirms the crystal has blue fluorescence emission. The thermal stability of the grown crystal was confirmed by TG/DTA study. The Hardness value was measured by using Vickers microhardness test. The dielectric properties were studied at room temperature. The grown crystal belongs to negative photoconductivity nature. The NLO study was carried out and compared with KDP. The optical behavior, including second harmonic generation (SHG), was investigated to study its NLO property.