Nanocrystal Thin Films Research Articles

Large Voltage Generation of Flexible Thermoelectric Nanocrystal Thin Films by Finger Contact

AbstractThis paper demonstrates that thermal energy radiated from a human finger can be converted efficiently into electricity by a nanocrystal (NC) thin film that substantially suppresses thermal conduction, but still allows electric conduction. The converting efficiencies of the chalcogenide NC thin films with dimensions 40 µm × 20 µm × 20 nm, prepared on flexible substrates by a solution process, are maximized by adjusting the NC size. A Seebeck coefficient of S = 1829 µV K−1, and a dimensionless thermoelectric figure‐of‐merit, ZT = 0.68 are achieved at ambient temperature for p‐ and n‐type NC thin films, respectively. A thermoelectric array consisting of p‐ and n‐type NC thin films generates a voltage of 645 mV for a temperature gradient of 10 K. Furthermore, the donut‐shaped pn array can generate a voltage of 170 mV from the heat supplied by an individual's finger.

Reflection of circularly polarized light and the effect of particle distribution on circular dichroism in evaporation induced self-assembled cellulose nanocrystal thin films

Evaporation induced self-assembled (EISA) thin films of cellulose nanocrystals (CNCs) have shown great potential for displaying structural colour across the visible spectrum. They are believed primarily to reflect left handed circularly polarised (LCP) light due to their natural tendency to form structures comprising left handed chirality. Accordingly the fabrication of homogenously coloured CNC thin films is challenging. Deposition of solid material towards the edge of a dried droplet, via the coffee-stain effect, is one such difficulty in achieving homogenous colour across CNC films. These effects are most easily observed in films prepared from droplets where observable reflection of visible light is localised around the edge of the dry film. We report here, the observation of both left and right hand circularly polarised (LCP/RCP) light in reflection from distinct separate regions of CNC EISA thin films and we elucidate how these reflections are dependent on the distribution of CNC material within the EISA thin film. Optical models of reflection are presented which are based on structures revealed using high resolution transmission electron microscopy (TEM) images of film cross sections. We have also employed spectroscopic characterisation techniques to evaluate the distribution of solid CNC material within a selection of CNC EISA thin films and we have correlated this distribution with polarised light spectra collected from each film. We conclude that film regions from which RCP light was reflected were associated with lower CNC concentrations and thicker film regions.

Crystalline phase transformation of colloidal cadmium sulfide nanocrystals

In this paper, we give a microscopic view concerning influence of the growth conditions on the physical properties of nanocrystals (NCs) thin films made of CdS, prepared using chemical bath deposition CBD technique. We show a crystalline phase transformation of CdS NCs from hexagonal wurtzite (W) structure to cubic zincblende (ZB) when the growth conditions change, particularly the solution pH values. This effect was confirmed using X-ray diffraction (XRD), transmission electron microscopy (TEM), optical absorption and photoluminescence (PL) measurements. The optical absorption spectra allow calculation of the bandgap value, [Formula: see text], where significant increase [Formula: see text]200 meV in the CdS bandgap when transforming from Hexagonal to Cubic phase was found.

Light emissions from a silicon nanocrystal thin film prepared by phase separation of hydrogen silsesquioxane

We report a facile method to prepare thin film of Si nanocrystals embedded SiO2 (Si-NC:SiO2) by annealing a photoresist of hydrogen silsesquioxane (HSQ) at 1100°C in nitrogen via a phase separation process. The spatial density, photoluminescence intensity, the photoluminescence efficiency and electroluminescence intensity of Si-NC of the sample made from HSQ, or HSQ sample, were 15.0, 5.5, 1.5 and 7.9 times as large as those of the sample made by a traditional method of annealing SiOx (1<x<2), or SiOx sample, respectively. Meanwhile, the turn-on voltage of electroluminescence of the HSQ sample was only 3.8eV, which was more than 2 times smaller than that of the SiOx sample. The results of this work may find application in developing high brightness Si light sources.

Engineering the surface chemistry of lead chalcogenide nanocrystal solids to enhance carrier mobility and lifetime in optoelectronic devices.

We introduce a stepwise, hybrid ligand-exchange method for lead chalcogenide nanocrystal (NC) thin films using the compact-inorganic ligand thiocyanate and the short organic ligand benzenediothiolate. Spectroscopic and device measurements show that hybrid exchange enhances both carrier mobility and lifetime in NC thin films. The increased mobility-lifetime product achieved by this method enables demonstration of optoelectronic devices with enhanced power conversion and quantum efficiency.

Fabrication of Cu2MSnS4 (M = Co2+, Ni2+) nanocrystal thin films and their application in photodetectors

Cu2CoSnS4 and Cu2NiSnS4 nanocrystal thin films and selenized films are prepared using a simple and low-cost solution process, and demonstrated a strong and reversible photoresponse performance.

Effect of Annealing Temperature and Spin Coating Speed on Mn-Doped ZnS Nanocrystals Thin Film by Spin Coating

ZnS:Mn nanocrystals thin film was fabricated at 300°C and 500°C via the spin coating method. Its sol-gel was spin coated for 20 s at 3000 rpm and 4000 rpm with metal tape being used to mold the shape of the thin film. A different combination of these parameters was used to investigate their influences on the fabrication of the film. Optical and structural characterizations have been performed. Optical characterization was analyzed using UV-visible spectroscopy and photoluminescence spectrophotometer while the structural and compositional analysis of films was measured via field emission scanning electron microscopy and energy dispersive X-ray. From UV-vis spectra, the wavelength of the ZnS:Mn was 250 nm and the band gap was within the range 4.43 eV–4.60 eV. In room temperature PL spectra, there were two emission peaks centered at 460 nm and 590 nm. Under higher annealing temperature and higher speed used in spin coating, an increase of 0.05 eV was observed. It was concluded that the spin coating process is able to synthesize high quality spherical ZnS:Mn nanocrystals. This conventional process can replace other high technology methods due to its synthesis cost.

Application of composition controlled nickel-alloyed iron sulfide pyrite nanocrystal thin films as the hole transport layer in cadmium telluride solar cells

The solution-processed colloidal NixFe1−xS2 nanocrystal based thin films serve as the hole transport layers in cadmium telluride (CdTe) solar cells.

Preparation and characterization of sphere-like Cu2SnS3 nanoparticles and their dropcasted thin films

Ternary sphere-like Cu2SnS3(CTS) semiconductor and 2D hexagonal sheets were synthesized via a simple solvothermal method using PVP as the surface ligand at two temperatures of 180 and 220 °C. The structural, morphological, and chemical compositions as well as optical properties of as-synthesized CTS particles were characterized using X-ray diffraction (XRD), Raman spectroscopy, energy dispersive X-ray spectrometry (EDS), field emission scanning electron microscopy (FESEM), and UV–Vis spectroscopy. The size of sphere-like particles and the side length of hexagonal sheets were within the range of 120–140 nm and 500 nm–, respectively. FESEM, XRD, and EDS were analyzed to investigate the mechanism of the morphological evolution of CTS particles. CTS particles showed proliferation of Sn atomic ratio, which is strongly sensitive to reaction temperature and, highly affects the increase of band gap energy from 1.36 to 1.53 eV due to generation metal defects and formation SnS2. The optical analysis via the transmittance and reflectance reveals that the band-gap energy of dropcasted CTS thin films decreases after annealing due to grain growth and change of chemical compositions. Photo-responses of CTS nanocrystal thin films indicated a considerable increase in the conductivity of the films under light illumination. All these results showed the potential of these films for solar cell applications.

Synthesis and Characterization of Cu3(Sb1–xAsx)S4 Semiconducting Nanocrystal Alloys with Tunable Properties for Optoelectronic Device Applications

A synthetic method for producing monodisperse Cu3(Sb1–xAsx)S4 nanocrystals (NCs) with highly tunable semiconducting properties is discussed. Additionally, significant improvement in collectible photocurrent is observed for arsenic-rich compositions, with a 10-fold improvement in photocurrent density from Cu3AsS4 NC thin films compared to the more well-studied Cu3SbS4 composition. Interestingly, this improvement in photocurrent is observed despite the increased optical band gap of the arsenic-rich compositions. These results suggest Cu3AsS4 and arsenic-rich compositions of Cu3(Sb1–xAsx)S4 may yield higher photocurrent densities for solar conversion devices. Furthermore, the simple and robust synthesis, scalable spray coating strategy, and highly tunable nature of the NCs provides a foundation encompassing numerous fields requiring inexpensive and earth-abundant semiconducting materials.

Designing highly conductive and stable silver nanocrystal thin films with tunable work functions through solution-based surface engineering with gold coating process

We fabricated highly conductive and stable silver nanocrystal (Ag NC) thin film electrodes with tunable work functions using an all-solution-based method involving sequential ligand exchange and gold (Au) coating processes. We studied the effect of the Au coating process on the ligand-exchanged Ag NC thin films and successfully demonstrated the formation of a thin Au layer on the surface of the Ag NCs. We investigated the morphological, structural, optical, chemical, and electrical properties of the Ag NC thin films before and after Au coating. The work function of the electrodes was precisely controlled by varying the treatment time and concentration of gold chloride trihydrate (HAuCl4·3H2O) solutions. The thermal stability of the Au coated Ag NC thin films in air was greatly enhanced, and an extremely low resistivity of 3.4 μΩcm was achieved. Taking advantage of this solution-based low-temperature process and using a minimal amount of Au for cost reduction, we fabricated all-NC-based high-performance flexible photodetectors.

High brightness formamidinium lead bromide perovskite nanocrystal light emitting devices

Formamidinium lead halide (FAPbX3) has attracted greater attention and is more prominent recently in photovoltaic devices due to its broad absorption and higher thermal stability in comparison to more popular methylammonium lead halide MAPbX3. Herein, a simple and highly reproducible room temperature synthesis of device grade high quality formamidinium lead bromide CH(NH2)2PbBr3 (FAPbBr3) colloidal nanocrystals (NC) having high photoluminescence quantum efficiency (PLQE) of 55–65% is reported. In addition, we demonstrate high brightness perovskite light emitting device (Pe-LED) with these FAPbBr3 perovskite NC thin film using 2,2′,2″-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) commonly known as TPBi and 4,6-Bis(3,5-di(pyridin-3-yl)phenyl)-2-methylpyrimidine (B3PYMPM) as electron transport layers (ETL). The Pe-LED device with B3PYMPM as ETL has bright electroluminescence of up to 2714 cd/m2, while the Pe-LED device with TPBi as ETL has higher peak luminous efficiency of 6.4 cd/A and peak luminous power efficiency of 5.7 lm/W. To our knowledge this is the first report on high brightness light emitting device based on CH(NH2)2PbBr3 widely known as FAPbBr3 nanocrystals in literature.

Interface Engineering in Solution-Processed Nanocrystal Thin Films for Improved Thermoelectric Performance.

Solution-processed PbTe nanocrystal (NC) thin films are ligand exchanged by ethylenediamine and then annealed at 400 °C for enhancement of NC coupling. To further improve the performance, heterostructures are introduced into the PbTe/PbS films. Significantly, an optimized ZT of ≈0.30 is successfully achieved at 405 K. This method opens an avenue toward thermoelectric thin film devices with high performance.

Optical and electrical properties of ZnO nanocrystal thin films passivated by atomic layer deposited Al2O3

While colloidal semiconductor nanocrystal (NC) is preferred for use in solution-based optoelectronic devices, the large number of surface defects associated with its high surface-to-volume ratio degrades the optimal performance of NC-based devices due to the extensive trapping of free carriers available for charge transport. Here, we studied a simple and effective strategy to control the degree of passivation and doping level of solution-deposited ZnO NC films by infilling with ultra-thin Al2O3 using an atomic layer deposition (ALD) technique. According to various spectroscopic, microstructural, and electrical analyses, the ALD-Al2O3 treatment dramatically reduced the number of surface trap states with high ambient stability while simultaneously supplied excess carriers probably via a remote doping mechanism. As a consequence, the field-effect transistors built using the ZnO NC films with ALD-Al2O3 treatment for an optimal number of cycles exhibited significantly enhanced charge transport.

Microscopic properties of a nanocrystal aluminum thin film during nanoimprint using quasi-continuous method

In this article, we perform a multi-scale simulation of nanoimprint on a single-crystal aluminum thin film using the quasi-continuous method. The effects of contact stiffness between the stamper, thin film, and imprint orientation on the microstructure of the nanocrystal aluminum are examined. Results show that for a smaller value of contact stiffness, plastic deformation occurs relatively late. However, early plastic deformation is found when the stamper imprints perpendicular to the plane (111). In addition, to explore the effect of offset on the microstructure of a single-crystal aluminum thin film during the multi-imprint process, we perform a secondary imprint with an imprint depth of 10Å. The effect of the first nanoimprint process on the secondary process can be found to be negligible when the offset is larger than 100Å. However, we do not obtain a good pattern for a 50-Å offset. Therefore, for simulation cases, we suggest that the appropriate offset between the two imprints is 100Å.

Near infrared to ultraviolet optical properties of bulk single crystal and nanocrystal thin film iron pyrite

We report optical properties of iron pyrite (FeS2) determined from ex situ spectroscopic ellipsometry measurements made on both a commercially available bulk single crystal and nanocrystalline thin film over a spectral range of 0.735–5.887 eV. The complex dielectric function, ε (E) = ε1 (E) + iε2 (E), spectra have been determined by fitting a layered parametric model to the ellipsometric measurements. Spectra in ε are modeled using a Kramers–Kronig consistent critical point parabolic band model involving seven critical points for the bulk single crystal and four critical points for the nanocrystalline film. Absorption coefficient spectra for both types of samples are also determined from ε. Critical point features in the nanocrystalline films are broader, have lower amplitude and lower energy critical points detected having a small blue shift when compared to the single crystal sample.

C60 nanocrystals thin film with controlled density

ABSTRACTIn this paper, C60 nanocrystals thin film with monocrystal thickness was prepared using a liquid-liquid interface. C60 nanocrystals dispersion prepared by reprecipitation method was added to n-hexane/water interface to form a thin film. The film was transferred onto an ITO substrate. Surface morphology of the film was studied by scanning electron microscope (SEM) and atomic force microscope (AFM) measurements. It was found that the density of the film increase with increasing the deposition number of layers with maintaining the monocrystal thickness.

Synthesis of colloidal NiO nanocrystals by a hot‐injection approach with a protecting ligand

We synthesize small and uniform NiO nanocrystals, which can be potential for hole‐transporting of organic solar cells and transparent conducting material, based on a protecting ligand of lithium stearate by a hot‐injection method. And the reaction mechanism is alcoholysis of metal carboxylate salts with the protection ligand of lithium stearate binding to the surfaces of NiO nanocrystals to prevent them reduction into Ni. We find that the hot‐injection method is an effective way to prepare narrow size distribution and small size of NiO nanocrystals. The relatively superior optical transparency and flat surface features of the NiO nanocrystal thin films are also obtained.

Majority Carrier Type Control of Cobalt Iron Sulfide (CoxFe1–xS2) Pyrite Nanocrystals

We report the colloidal synthesis, characterization, and electronic property control of compositionally varied CoxFe1–xS2 cubic pyrite nanocrystals (NCs) and thin films formed from solution. Using drop-cast NC thin films, we demonstrate the relationship between the material composition and the majority carrier type of the nanocrystalline thin films. Measurements of the majority carrier type as a function of NC composition indicate that CoxFe1–xS2 NC thin films change from p-type to n-type between x = 0.16 and x = 0.21. Additional characterization to confirm the crystallinity, composition, size, and shape was performed using powder X-ray diffraction, Raman spectroscopy, energy dispersive X-ray spectroscopy, and scanning electron microscopy. The observed n-type behavior which accompanies the substitution of Co for Fe in these cubic pyrite nanostructures agrees with previous reports of n-type behavior occurring at even very low concentration Co doping of iron pyrite. The ability to prepare n- or p-type pyrit...

Fabrication and Characterization of TiO&lt;sub&gt;2&lt;/sub&gt;-Doped ZnAl&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;4&lt;/sub&gt; Nanocrystals via Sol-Gel Method for GPS Antenna

The Zn(1-x)TixAl2O4 (x = 0.00, 0.05, 0.15 &amp; 0.25) nanocrystals thin films were prepared by sol-gel method. The properties of Zn(1-x)TixAl2O4 were investigated by X-ray diffraction (XRD), Atomic Force Microscope (AFM), Fourier transform infrared spectra (FTIR) and (UV-Vis). By indexing the XRD patterns, we identified three structural types which is ZnAl2O4, anatase and rutile. The addition of TiO2 increased the crystallite size from 14.65 to 25.25 nm. The direct band gap was found to be around 3.35 to 3.84 eV. The addition of TiO2 increased the crystallite size, surface roughness, and lattice parameters of the resultant films, evidently affecting their density and dielectric constant (). The thin films were characterized in the certain frequency to determine the using LCR spectrometer. The and density value of the Zn(1-x)TixAl2O4 films increase linearly from 8.56 to 13.48 and 4.60 to 4.70 g/cm3 with the increasing of x value, respectively. Based on the material analysis and microwave antenna theory, GPS patch antennas were fabricated using the Zn(1-x)TixAl2O4 material. The fabricated GPS antenna with the highest (13.48) material exhibits the smallest size of antenna which is 7.45 cm2. The performances and the operating frequencies were measured using a PNA series network analyzer. The result showed that all patch antennas operate at frequency of 1.570 GHz. The GPS patch antenna fabricated from Zn0.25Ti0.75Al2O4 showed an excellent combination of return loss (-29.6 dB), smallest size (7.85 cm2), and wide bandwidth (195 MHz). All fabricated antennas are meets the requirements of GPS applications.