Nanocrystalline Semiconductor Research Articles

Mullite-type Bi2Mn4O10 thin films prepared by a PVA-assisted sol–gel method

Mullite-type Bi2Mn4O10 is an attractive material for several potential applications, such as energy conversion and storage, data storage, batteries and photoelectrochemical devices. Bi2Mn4O10 thin films were prepared by sol–gel from a simple and highly stable precursor solution, based on the corresponding metal nitrate salts, acetic acid and polyvinyl alcohol. The films were deposited by dip-coating onto FTO substrates and dried at 350 °C; the dipping-drying cycle was repeated six times and the films were finally sintered at 600 °C. The films were characterized using GIXRD, revealing an excellent match with the ICDD database and previous studies. The UV–Vis measurements showed a direct band gap of 1.78 eV for the films. Raman spectroscopy permitted the identification of the Ag, B1g and B2g vibrational modes. The electrical properties via Hall effect measurements showed that the Bi2Mn4O10 thin films exhibit an n-type semiconductor behavior. Finally, the FESEM micrographs and AFM images revealed homogeneous, nanostructured thin films. In summary, through this comprehensive characterization, it was observed that high quality, nanocrystalline semiconductor thin films of Bi2Mn4O10 can be produced using this simple sol–gel method.

Metal Halide Perovskite Light-Emitting Transistor with Tunable Emission Based on Electrically Doped Semiconductor Nanocrystal-Based Microcavities

Electroluminescence of metal halide perovskites has been widely reported via the fabrication and optimization of light-emitting diodes and light-emitting transistors. Light-emitting transistors are particularly interesting owing to the additional control of the gate voltage on the electroluminescence. In this work, the design of a microcavity, with a defect mode that can be tuned with an applied voltage, integrated with a metal halide light-emitting transistor is shown. The optical properties of the device have been simulated with the transfer matrix method, considering the wavelength-dependent refractive indexes of all the employed materials. The tunability of the microcavity has been obtained via the employment of doped semiconductor nanocrystalline films, which show a tunable plasma frequency and, thus, a tunable refractive index as a function of the applied voltage. Consequently, the tunability of the electroluminescence of the metal halide perovskite light-emitting transistor has been demonstrated.

Effective role of vibrational annealing in enhancing room temperature physical properties of Co doped ZnO nanoparticles by using novel TVA technique

Well-annealed nanocrystalline semiconductor materials in pellet or thin film form, composed of small grains with a high surface-to-volume ratio, are considered as promising candidates for multiple device applications. However, no such method is available yet that can control the grain size of such samples during heat treatment. In this present work, a novel thermo-vibrational annealing (TVA) setup for processing nanocrystalline materials was designed and fabricated in-house. Here both thermal and mechanical vibrational energy were simultaneously applied to the sample during annealing, followed by vibrational dry quenching in the presence of an inert gas environment. Co-doped ZnO pellets were synthesized by chemical route pyrophoric method followed by annealing at 800 °C for 4 h using this set up. For comparison, one pellet was annealed with vibration with calibrated vibration at a speed of 360 rpm, and another pellet was annealed without vibration. X-ray diffraction (XRD), Raman and Fourier transform Infrared (FTIR) spectroscopic studies showed the formation of a single crystalline phase with hexagonal wurtzite structure of both the samples and the proper incorporation of Co2+ ions into the ZnO matrix without any impurity phases, which was also confirmed by energy dispersive X-ray (EDX) spectroscopy studies. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM) studies confirmed the decrease in average grain and particle size of the sample when processed with the TVA setup. Ultraviolet–Visible spectra showed that the bandgap energy of the Co-doped ZnO sample varies inversely with particle size. Hall-effect measurements demonstrated the semiconducting nature of both samples with n-type charge carriers. The presence of Co2+ ions in the wurtzite ZnO lattice was confirmed by X-ray photoelectron spectroscopy (XPS). The magnetic properties were studied using a SQUID magnetometer, which confirmed that a Co doped ZnO sample annealed with the TVA technique exhibit better room temperature ferromagnetism properties compared to sample annealed without vibration. Based on these studies, it was believed that the sample annealed by mechanical vibrational energy with vibrational dry quenching has a great influence on several physical properties of any material and hence Co doped ZnO sample processed with this TVA technique can be used as a promising candidate for multiple sensor and magnetic storage devices.

Enzyme assembly on nanoparticle scaffolds enhances cofactor recycling and improves coupled reaction kinetics.

Enzyme activity can be many times enhanced in configurations where they are displayed on a nanoparticle (NP) and this same format sometimes even provides access to channeling phenomena within multienzyme cascades. Here, we demonstrate that such enhancement phenomena can be expanded to enzymatic cofactor recycling along with the coupled enzymatic processes that they are associated with. We begin by showing that the efficiency of glucose driven reduction of nicotinamide adenine dinucleotide (NAD+ → NADH) by glucose dehydrogenase (GDH) is enhanced ca. 5-fold when the enzyme is displayed on nanocrystalline semiconductor quantum dots (QDs) which are utilized as prototypical NP materials in our experimental assays. Coupling this enzymatic step with NADH-dependent lactate dehydrogenase (LDH) conversion of lactate to pyruvate also increases the latter's rate by a similar amount when both enzymes were jointly incorporated into self-assembled QD-based nanoclusters. Detailed agarose gel mobility assays and transmission electron microscopy imaging studies confirm that both tetrameric enzymes assemble to and crosslink the QDs into structured nanoclusters via their multiple-pendant terminal (His)6 sequences. Unexpectedly, control experiments utilizing blocking peptides to prevent enzyme-crosslinking of QDs resulted in even further enhancement of individual enzyme on-QD kinetic activity. This activity was also probed revealing that 200-fold excess peptide/QD addition enhanced individual GDH and LDH on-QD kcat a further 2- and 1.5×, respectively, above that seen just by QD display to a maximum of ∼10-fold GDH enhancement. The potential implications for how these enzyme kinetics-enhancing phenomena can be applied to single and multi-enzyme cascaded reactions in the context of cofactor recycling and cell-free synthetic biology are discussed.

Thermal Stability of Semiconductor Nanocrystal Solids: Understanding Nanocrystal Sintering and Grain Growth

Nanomaterials are naturally metastable with respect to bulk solids. This raises the very important fundamental problem of their morphological stability, especially when nanoscale crystallites are touching or nearly touching each other, such as in thin-film devices. In some cases, nanostructuring must be preserved under operational conditions (e.g., in quantum dot LEDs, lasers, photodetectors, and nanogranular thermoelectric devices). In other cases, we use nanocrystalline particles as precursors to a material with large crystalline grains and aim to sinter them as efficiently as possible (e.g., in polycrystalline thin-film solar cells). We carried out a systematic study of sintering and grain growth in materials composed of various sub-10 nm semiconductor grains. The boundaries between individual semiconductor grains have been chemically engineered using inorganic surface ligands. We found that the early stages of sintering and grain growth of nanocrystalline semiconductors are controlled by the ion mobility at the nanocrystal surfaces, while the late stages of grain growth are controlled by the mobility of the grain boundaries. This appears to be a general phenomenon for semiconductor nanocrystals, and it leads to several interesting and counterintuitive trends. For example, III–V InAs nanocrystals are generally much more resilient against sintering and grain growth compared to II–VI CdSe nanocrystals even though bulk CdSe has significantly higher melting point temperature than InAs (1268 °C vs 942 °C). Grain growth can be dramatically accelerated when coupled to solid−solid phase transitions. These findings expand our toolbox for rational design of nanocrystal materials for different applications.

Laser radiation effect on microstructural, linear, and nonlinear optical characteristics of Sn0.04Co0.06O2 nanostructured films

The influence of He-Ne laser radiation on linear and nonlinear optical characteristics of Co-doped SnO2 (Sn0.04Co0.06O2) thin film was studied before and after irradiation. 22 mW (633 nm) He-Ne laser beam was employed to irradiate the film with different exposure times (Texpo. = 1, 2, 5, 15, 20, 40, and 60 min). As grown and irradiated films were examined by X-ray diffraction (XRD), an atomic force microscope (AFM) was integrated with UV-VIS–NIR spectrophotometer. XRD pattern analysis showed that the crystallite size decreased as the exposure increased to 5 min, however, a further increase in Texpo from 15 to 60 min increased the crystallite size. Analysis of AFM micrographs shows that the roughness of the surface and root mean square roughness of the film depend on the increase in the laser exposure time. The optical studies of irradiated films show that the optical energy gap Eg decreases as the exposure time increases to 5 min and then raises as the film is irradiated for up to 60 min. This reduction was attributed to the elimination of the additional oxygen from the film, and the increase of Eg is explained by the Burstein–Moss effect. Swanepoel’s envelope method is used to calculate the linear optical parameters of the irradiated film from the light transmission spectrum, such as refractive index n, and extinction coefficient k. The results reveal that the global behavior of n and k increases with exposure time. The oscillator parameters of the irradiated film of different exposure times were evaluated from the single-oscillator Wemple–DiDomenico model, which was then used to calculate the nonlinear optical parameters such as n2 the nonlinear refractive index, and χ(3) the third-order nonlinear optical susceptibility. It is concluded that the He-Ne laser irradiation with different exposure times enhances the optoelectronic properties of Sn0.04Co0.06O2 nanocrystalline semiconductor thin film

Advances in Electrostatic Spinning of Polymer Fibers Functionalized with Metal-Based Nanocrystals and Biomedical Applications.

Considering the metal-based nanocrystal (NC) hierarchical structure requirements in many real applications, starting from basic synthesis principles of electrostatic spinning technology, the formation of functionalized fibrous materials with inorganic metallic and semiconductor nanocrystalline materials by electrostatic spinning synthesis technology in recent years was reviewed. Several typical electrostatic spinning synthesis methods for nanocrystalline materials in polymers are presented. Finally, the specific applications and perspectives of such electrostatic spun nanofibers in the biomedical field are reviewed in terms of antimicrobial fibers, biosensing and so on.

Properties and White Light Photoresponses of CdSe Colloidal Nanoparticles

II–VI nanocrystalline semiconductor materails provide a wide range of applications in electronics, optoelectronics and photonics. The present paper reports the preparation of colloidal Cadmium selenide (CdSe) nanoparticles and study their optical properties. (CdSe) was produced by a chemical route procedure, with the ligand tri sodium citrate (TSC) serving as a complexing agent. Cadmium acetate is used as a Cd source, and sodium selenide is used as a Se precursor. The morphological and optical properties of the produced nanoparticles were determined using a Transmission electron microscopy (TEM) and Ultraviolet – Visible spectroscopy (UV-VIS).Thin films of CdSe capped with are deposited on the glass and aluminum substrates . AL/ CdSe/Ag MSM photodetectors have been fabricated and the electrical properties were measured in dark and light. The output data display that the photo currents of all diodes are increased with light. The fact that the films had dark electrical conductivity and photoconductivity indicates that they were semiconducting materials.

N-Type doping of a solution processed p-type semiconductor using isoelectronic surface dopants for homojunction fabrication

The p-n junction is one of the fundamental requirements for a practical semiconductor-based electronic device. Designing a heterojunction comprising of dissimilar p-type and n-type semiconductors calls for careful energy level considerations, both when selecting the semiconductor materials as well as the metal contacts. A homojunction based on a single semiconductor simplifies this task, as energy levels of the p-type and n-type materials are already fairly similar, allowing for easier selection of contacts. Traditionally, homojunctions rely on doping of a bulk semiconductor to achieve p- and n-type transport through controlled addition of aliovalent dopants via energy-intensive processes such as ion implantation or thermal annealing. Exact control of doping in nanocrystalline semiconductors is significantly more challenging, due to self-purification effects. However, owing to their large surface areas, surface moieties can be utilized to both dope the nanostructures as well as tune their energy levels. In this report, we present a facile technique based on an isoelectronic surface dopant in order to achieve p- and n-type materials based on the same semiconductor. We show that thin p-type colloidal Bi2Te3 nanowires can be switched to n-type through surface functionalization, thus increasing the availability of new nanocrystalline solution-processable p-n homojunctions.

Optical, morphological, electrical properties and white light photoresponse of CdSe nanoparticles

ABSTRACT II–VI nanocrystalline semiconductors offer a wide range of applications in electronics, optoelectronics, and photonics. The study of the morphological, electrical, and optical properties of Cadmium selenide (CdSe) nanoparticles prepared with tri ethanolamine (TEA) as a complex agent is reported in the paper. Cadmium acetate as a Cd source and (Na2SeO3) as a Se precursor. Transmission electron microscope (TEM), field emission scanning electron microscope (FESEM), energy dispersive x-ray (EDX), and ultraviolet-visible spectroscopy (UV-VIS) were used to characterise the synthesised nanoparticles for their optical, compositional, morphological, and electrical properties. The film of CdSe was deposited on the glass substrate. The electrical characterisation of CdSe thin films at various temperatures dark electrical conductivity and photoconductivity reveal that the films were semiconducting in nature, indicating the suitability of these films for photo sensor applications.

Development of methods for studying intergranular surface features in semiconductor heterogeneous polycrystals of bismuth-antimony tellurides with the imposition of electric and deformation fields

The article discusses the technique and tools for studying the role of the effective density of electronic surface states innanocrystalline semiconductor films when cyclic deformation is applied directly from the analysis of experimental data. The surface electron states play the role of recombination and trapping centers depending on the number of carriers, the electron capture section and hole, the concentration of surface states of their type and energy position. To determine the effective density of surface states, we found both a change in the Fermi level and a change in the density of the effective surface charge. The effective density of electronic surface states was determined from the measured variations in the active resistance and capacity of nanocrystalline films of bismuth-antimony tellurides upon the application of irreversible deformation, and its strain dependence was found. From data analysis, one can judge the irreversibility of heterogeneous structures during deformation, that is, the electronic structure of a nanocrystalline semiconductor film changes greatly when cyclic deformation is applied.

Thin nanocrystalline semiconductor films as selective chemical sensors for ammonia, acetone and other donors

Studied are the films of variously doped polycrystalline n-semiconductors (ZnO, SnO2) as selective sensitive elements (SE) of chemical sensors for various gases and vapours (ammonia, acetone, propane, ethanol, hexane, solvent, turpentine etc.) in artificial air. It has been revealed that their conductivity changes under temperature modulation makes possible data processing which identifies the impurities above. This processing is based on nonlinear regression estimation of so called principal parameters which set is unique for every concentration of every of the gases/vapours.

Nanoenabled Bioelectrical Modulation.

ConspectusStudying the formation and interactions between biological systems and artificial materials is significant for probing complex biophysical behaviors and addressing challenging biomedical problems. Bioelectrical interfaces, especially nanostructure-based, have improved compatibility with cells and tissues and enabled new approaches to biological modulation. In particular, free-standing and remotely activated bioelectrical devices demonstrate potential for precise biophysical investigation and efficient clinical therapies. Interacting with single cells or organelles requires devices of sufficiently small size for high resolution probing. Nanoscale semiconductors, given their diverse functionalities, are promising device platforms for subcellular modulation. Tissue-level modulation requires additional consideration regarding the device’s mechanical compliance for either conformal contact with the tissue surface or seamless three-dimensional (3D) biointegration. Flexible or even open-framework designs are essential in such methods. For chronic organ integration, the highest level of biocompatibility is required for both the materials and device configurations. Additionally, a scalable and high-throughput design is necessary to simultaneously interact with many individual cells in the organ. The physical, chemical, and mechanical stabilities of devices for organ implantation may be improved by ensuring matching of mechanical behavior at biointerfaces, including passivation or resistance designs to mitigate physiological impacts, or incorporating self-healing or adaptative properties.Recent research demonstrates principles of nanostructured material designs that can be used to improve biointerfaces. Nanoenabled extracellular interfaces were frequently used for either electrical or remote optical modulation of cells and tissues. In particular, methods are now available for designing and screening nanostructured silicon, especially chemical vapor deposition (CVD)-derived nanowires and two-dimensional (2D) nanostructured membranes, for biological modulation in vitro and in vivo. For intra- and intercellular biological modulation, semiconductor/cell composites have been created through the internalization of nanowires, and such cellular composites can even integrate with living tissues. This approach was demonstrated for both neuronal and cardiac modulation.At a different front, laser-derived nanocrystalline semiconductors showed electrochemical and photoelectrochemical activities, and they were used to modulate cells and organs. Recently, self-assembly of nanoscale building blocks enabled fabrication of efficient monolithic carbon-based electrodes for in vitro stimulation of cardiomyocytes, ex vivo stimulation of retinas and hearts, and in vivo stimulation of sciatic nerves.Future studies on nanoenabled bioelectrical modulation should focus on improving efficiency and stability of current and emerging technologies. New materials and devices can access new interrogation targets, such as subcellular structures, and possess more adaptable and responsive properties enabling seamless integration. Drawing inspiration from energy science and catalysis can help in such progress and open new avenues for biological modulation. The fundamental study of living bioelectronics could yield new cellular composites for diverse biological signaling control. In situ self-assembled biointerfaces are of special interest in this area as cell type targeting can be achieved.

PbSe Quantum Dots for X-ray and γ-Ray Sensors

Nanocrystalline (NC) semiconductors exhibit exploitable properties, such as a tunable energy band gap and multiexciton generation, which arise due to strong quantum confinement. Lead selenide (PbSe), with a relatively large Bohr radius of 46 nm, possesses an extensive literature of organic synthesis routes with which one can explore the strong confinement regime in quantum dots, nanorods, and nanowires. The intrinsically high charge mobility combined with a high atomic number and density of the lead chalcogenides makes them attractive for sensing applications with highly penetrating quanta, such as X-rays and γ-rays. In sensing architectures, the exploitation of these properties for each individual nanocrystallite is hampered by the need to transport the charge carriers throughout the active volume, a motion that can be retarded by energetic surface barriers typically in the form of insulating oxides. Here, we prevent surface oxidation through the fabrication of PbSe NCs via tris(diethylamino)phosphine and a purified selenium precursor, a process that results in PbSe NCs that are chemically and optically stable for at least 1.6 years. If one endeavors to measure high-energy quanta, then the micrometer-scale stopping layers typical of optical photon sensors are insufficient and one must therefore find methods to interconnect the NCs through millimeter- to centimeter-scale thicknesses. Here, we report on the growth of millimeter-scale PbSe colloidal solids that are directly grown within the NC solution. Finally, in contrast to optical photon sensors that typically measure photocurrent, high-energy particle and photon sensors that measure the energy from each interacting quantum are typically hampered by the solid’s thermal noise and the counting statistics associated with discretizing a single quantum into a finite number of information carriers. We show that one can exploit the weaker phonon–electron coupling in nanocrystalline materials to produce room-temperature sensors of X-rays and γ-rays that have comparable resolution to state-of-the-art high-purity germanium detectors. The results thus suggest that multiexciton generation in nanocrystals can be profitably employed in sensing applications that target quanta that create hot-carrier populations that are well above the band gap.

Linear, non-linear optical properties and magnetic studies of spray pyrolysis nanocrystalline Sn1-xCoxO2 films for multifunctional optoelectronic and spintronic applications

Abstract In this paper, the structural, morphological, linear & non-linear optical, and magnetic properties of spray pyrolysis nanostructured Sn1-xCoxO2 thin films (0 ≤ x ≤ 0.08) were studied employing X-ray diffraction (XRD), atomic force microscope (AFM), spectrophotometric, and vibrating sample magnetometer (VSM) techniques, respectively. A tetragonal type structure was observed for films without any sign of impurities. XRD with Rietveld refinement analysis further revealed an increase in lattice parameters and unsystematic changes in crystallite size. AFM micrograph analysis shows that the surface roughness and root mean square roughness of the film are improved with increasing Co doping. Moreover, the spectrophotometry results indicate a direct optical transition for the film with an energy band gap reduces from 3.8 eV (x = 0) to 3.34 eV (x = 0.07) due to the rise in structural disorder and the increase in the density of localized states. The optical constants are calculated from the light transmission spectrum using the envelope method of Swanepoel. It is seen that the overall behavior of the refractive index n(λ) and extinction coefficient k(λ) increase with increasing of Co content. The dispersive oscillator parameters estimated from the single-oscillator Wemple–DiDomenico model were used to determine the non-linear optical parameters, such as the third-order non-linear optical susceptibility χ(3) and non-linear refractive index n2. Finally, intrinsic room temperature ferromagnetism is observed, which is interpreted based on the bound magnetic polarons model. In conclusion, Co doping in SnO2 host semiconductor nanocrystalline films enhances their optical and magnetic characteristics, making it a potential candidate for multifunctional applications like optoelectronics and spintronics devices.

The LSPR regulation of TiO2: W nanocrystals and its application in enhanced upconversion luminescence

The localized surface plasmon resonance (LSPR) absorption peaks of semiconductor nanocrystals are mainly concentrated in the infrared band, and the absorption characteristics can be controlled by the amount of element doping. The coupling of upconversion nanocrystals (UCNPs) and semiconductor nanocrystals can improve the upconversion luminescence (UCL) of rare-earth ions. Here, the LSPR absorption and morphology of the semiconductor nanocrystalline TiO2: W were adjusted by using ammonium fluoride during synthesis. Significant absorption enhancement of TiO2: W in the near-infrared region was obtained to enhance the UCL of NaYF4: Yb3+, Er3+. The Glass/NaYF4: Yb3+, Er3+/TiO2: W@SiO2 layered structure films were fabricated through spin coating. Compared with Glass/NaYF4: Yb3+, Er3+, the green and red lights of the Glass/NaYF4: Yb3+, Er3+/TiO2: W@SiO2 films were enhanced by 15.9 and 17.8 times, respectively. The UCL enhancement of Glass/NaYF4: Yb3+, Er3+/TiO2: W@SiO2 was derived from the LSPR property of TiO2: W through the enhancement of the excitation. The present work is important for possible applications of these layered structures as biomarkers, photocatalysts, flexible materials, and photoluminescence display panels.

Restructuring and interlayer phenomenon on tandem architecture to improve organic solar cell behaviour

Tandem solar cells have spurred much interest among the researchers because they provide an efficient way to harvest an extensive spectrum of solar emission by integrating two or more subcells with varying bandgap. However, the understanding related to working phenomenon of the interfacial layer integrating the absorption layers in a tandem solar cell lags and these interlayer plays a significant role in achieving higher efficiency. In this manuscript, we inquire the aspect of the p-type and n-type conducting region creating an interlayer and several significant factors of the tandem cell-like interfacial engineering, optical optimisation and incident photon to current efficiency and PCE characteristics. Closer investigation of the interlayer, consisting of the n-type nanocrystalline semiconductor materials like TiO2/ZnO/SnO2 and a p-type hole mobility layer explains the ohmic contact phenomenon through the metal/semiconductor interface. Here, we determine that a higher efficiency tandem solar cells with a power conversion efficiency of about 8.2% can be attained through an effective interlayer attribute and different absorption bands for electron generation. The understanding experienced in this manuscript can be constructive in enhancing the efficiency of the tandem solar cells further with wider bandgap polymers.

Understanding the structure, bonding and reactions of nanocrystalline semiconductors: a novel high-resolution instrumental method of solid-state synchronous luminescence spectroscopy.

This Perspective Article describes the recent advancements in studies of nanocrystalline metal oxides using a novel ultra-high resolution method, solid-state synchronous luminescence spectroscopy (SS-SLS). Semiconductors notably include titanium dioxide and these studies shed light on the detailed electronic structure, composition, and their reactions. First, we critically discuss the limitations of the major existing non-spectroscopic and spectroscopic methods of characterization of electronic structure of nanocrystalline semiconductors and insulators. Second, we describe the foundations and the setup of SS-SLS as an enhanced-resolution, facile, non-contact, non-destructive, and highly capable method of studies of nanomaterials. Third, the following insights are featured which are obtained by SS-SLS, but are not available by other methods: (a) detection of traps of electric charge (specific mid-gap states); (b) discrimination between "surface" and "bulk" sites; (c) in situ studies of composite nanomaterials and mechanisms of reactions, (d) the derivative SS-SLS for accurate determination of energies of absorption and emission. The specific advantages of SS-SLS versus other methods and in direct comparison with "conventional" photoluminescence spectroscopy are highlighted. Finally, new opportunities and challenges of SS-SLS are presented. SS-SLS is an advanced spectroscopic method with significant potential to aid academia and industry in studies of chemo-sensing, photocatalysis, optoelectronic materials, applied surface science, development of instrumental analysis, and studies of mechanisms of surface and "bulk" chemical reactions.

Thermopower Enhancement of Rutile-type SnO2 Nanocrystalline Using Facile Co-Precipitation Method

Metal oxide semiconductor has attracted so much attention due to its high carrier mobility. Herein, thermoelectric study of nanocrystalline SnO2 through a simple co-precipitation method is conducted to enhance the Seebeck coefficient (S). X-ray diffraction, thermogravimetric analysis (TGA), resistivity (r), Seebeck coefficient (S), and power factor (PF) measurements are conducted to analyze the thermoelectric properties of the material. The measurements show that there are two interesting results, which are the unusual resistivity behavior and the high value of the S. Resistivity behavior shows a non-reflective intermediate semiconductor-metals behavior where the turning point occurs at 250 o C. This behavior is strongly correlated to the surface oxide reaction due to annealing temperature. The maximum S likely occurs at 250 ºC, since the curve shows a slight thermopower peak at 250 ºC. The value of the S is quite high with around twenty times higher than other publications about SnO2 thermoelectric material, this happens due to the bandgap broadening. The energy gap of SnO2 calculated using density functional theory (DFT), which was performed by Quantum Espresso 6.6. The result shows that there is a broadening energy gap at different momentum or wave factor. Nanocrystalline semiconductors material is giving an impact to increase the width of bandgap due to quantum confinement and could enhance the thermopower especially in SnO2 nanocrystalline

New synthesis route to kesterite Cu2ZnSnS4 semiconductor nanocrystalline powders utilizing copper alloys and a high energy ball milling-assisted process

Presented is a new mechanochemically assisted synthesis route to kesterite Cu2ZnSnS4 semiconductor for perspective photovoltaic applications. Initially, the combined copper Cu, zinc Zn, and tin Sn powders in atomic ratio 2:1:1 were subjected to high energy ball milling in a planetary ball mill under the highest reported to date rotation speeds of 900 and 1000 rpm to yield a definite nanopowder mixture of the copper intermetallic phases, i.e., γ Cu5Zn8-type and η’ Cu6Sn5-type of non-stoichiometric compositions. Upon addition of a suitable amount of sulfur S, the reactive milling was continued affording a pure sphalerite-type cubic phase tentatively called a pre-kesterite. As expected from our earlier research, this by-product did not exhibit any 65Cu and 119Sn MAS NMR spectra or clearly-defined absorption in the UV–vis range. The disordered and very much random Cu-Zn-Sn metal site occupation of all metal layers in the structure is proposed to account for the observed properties of pre-kesterite. The subsequent annealing under argon at temperatures around 500−550 °C resulted in the cubic phase conversion to the tetragonal phase of kesterite, prevailingly, by metal center reconfiguration into distinct Cu-Sn and Cu-Zn layers accompanied by crystal growth/particle recrystallization. The micro-Raman spectra were consistent with similar bonding and lattice phonon characteristics in both related materials. The XPS data confirmed the same oxidation states and comparable energy level characteristics of the constituent elements. The annealed nanocrystalline powders showed the anticipated 65Cu and 119Sn MAS NMR features, while supporting remnant paramagnetism, and UV–vis semiconducting characteristics of the target kesterite.