Nanocrystal Thin Films Research Articles

Effect of Organic Nano-Components on the Thermoelectric Properties of Sb2te3 Nanocrystal Thin Film

In this work, organic CH3NH3I nano-components were successfully introduced into Sb2Te3 nanocrystal thin film via using the layer-by-layer evaporation growth method. The thermoelectric properties, including the electrical conductivity, Seebeck coefficient, thermal conductivity and the estimated figure of merit were investigated. The experimental results indicated that the introduced organic nano-components are beneficial to improving the Seebeck coefficient of the Sb2Te3 nanocrystal thin film, resulting in over 100% enhancement of figure of merit. These results prove our hypothesis that the organic hybrid can greatly enhance the thermoelectric property of the Sb2Te3 thin film based on the energy-filtering effect.

Transferrable thin film of ultrasonically exfoliated MoSe2 nanocrystals for efficient visible-light photodetector

Abstract Two-dimensional transition metal dichalcogenides (2D TMDC's) materials are the impactful candidates in nanotechnology for diverse applications such as electrocatalyst, sensing and energy storage. Many efforts have been accomplished to solve the problem of green production with the scaling of TMDCs materials. Here we demonstrated the two-step method including electrophoresis deposition (EPD) and direct transfer of MoSe2 nanocrystals (NCs) thin film for fabricating visible-light photodetector. High-resolution transmission electron microscopy and atomic force microscopy show that the liquid exfoliated MoSe2 NCs exhibits good crystalline nature with multi-layered thickness. Raman spectra, UV–Visible spectra, and X-ray photoelectron spectroscopy confirm the presence of 2H-phase of MoSe2 NCs. EPD deposited thin film is detached by dipping into the water and transferred onto the etched ITO substrate. Ag/MoSe2 NCs/ITO photodetector is fabricated which demonstrated the excellent performance in the visible range and demonstrate remarkable values of responsivity, EQE (%) and specific detectivity of 566 mAW−1, 155% and 3.69 × 1011 Jones. Considering our results, the fabrication technique along with the photoresponse application of TMDC NCs presented here is the innovative and efficient way for developing the large-area optoelectronic devices.

Property engineering through nanomaterial chemical transformation of colloidal nanocrystal thin films

Herein, we introduce a unique nanomaterial chemical transformation (NCT) process whereby selenium (Se) anion is introduced to colloidal silver (Ag) nanocrystals (NCs) to create nanostructured silver selenide (Ag2Se). We present a complete suite of material characterizations, including chemical, structural, optical, and electrical characterizations to understand the details of the transformation process. The Ag2Se thin-films obtained through the NCT process exhibit degenerately-doped n-type semiconductor characteristics with high carrier mobility. We discuss the chemical mechanism that drives the material transformation and elucidates the origin of doping in these semiconducting thin-films. We also demonstrate the robustness of Ag2Se thin-films toward mechanical strain and temperature cycling stress for flexible device applications. We believed that the NCT process demonstrated here is of wide applicability to other material systems which can open up new avenues for solid-state chemistry and device engineering research.

Dopants and Traps in Nanocrystal-Based Semiconductor Thin Films: Origins and Measurement of Electronic Midgap States

Different models are proposed to explain the origin of electronic midgap states that limit charge transport in nanocrystal thin films. Here, we investigate midgap states using optoelectronic and electrochemical techniques, supported by theoretical calculations. We find that charge trapping in PbS NC thin films can be explained by (1) nanocrystal dimerization and (2) charging of doped nanocrystals. In these models, there are no states within the band gap of individual nanocrystals; the nanocrystals themselves act as trap states. These findings allow us to formulate strategies to improve transport by increasing free carrier densities while reducing the number density of trap states.

Low-temperature plasma enhanced atomic layer deposition of large area HfS2 nanocrystal thin films**Project supported by the National Key Research and Development Program of China (Grant No. 2018YFB2200103).

Hafnium disulfide (HfS2) is a promising two-dimensional material for scaling electronic devices due to its higher carrier mobility, in which the combination of two-dimensional materials with traditional semiconductors in the framework of CMOS-compatible technology is necessary. We reported on the deposition of HfS2 nanocrystals by remote plasma enhanced atomic layer deposition at low temperature using Hf(N(CH3)(C2H5))4 and H2S as the reaction precursors. Self-limiting reaction behavior was observed at the deposition temperatures ranging from 150 °C to 350 °C, and the film thickness increased linearly with the growth cycles. The uniform HfS2 nanocrystal thin films were obtained with the size of nanocrystal grain up to 27 nm. It was demonstrated that higher deposition temperature could enlarge the grain size and improve the HfS2 crystallinity, while causing crystallization of the mixed HfO2 above 450 °C. These results suggested that atomic layer deposition is a low-temperature route to synthesize high quality HfS2 nanocrystals for electronic device or electrochemical applications.

Solution-processed sphere-like Cu2ZnSnS4 nanoparticles for solar cells: effect of oleylamine concentration on properties

Quaternary Cu2ZnSnS4 (CZTS) nanoparticles of high quality have been synthesized utilizing rotary evaporation technique. The volume of the solvent, oleylamine (OLM), was found to affect the morphological, structural and electrical characteristics of the synthesized structures. The materials were characterized using various analytical techniques. XRD patterns and Raman measurements reveal that CZTS nanoparticles exist in a crystalline state with a kesterite structure. For OLM of 6 mL, transmission electron microscopy reveals the formation of spherical CZTS nanoparticles. Scanning electron microscopy analysis of the nanocrystal thin films clearly shows a crack-free, uniform-grain thin film with a particle size in the range between 1 and 2 µm. Ultraviolet–visible–near infrared (UV–Vis–NIR) spectra showed a direct band gap of 1.47 eV, which is close to the optimum value required for photovoltaic applications. The current synthetic strategy is rapid and simple, and it can be utilized for commercial application. Solar cells were built using the structure glass/Mo/CZTS/CdS/i-ZnO/AZO/Ag and were found to exhibit a conversion efficiency of about 2%.

Ceria thin film memristive device by magnetron sputtering method

CeO2-x nanocrystal (NC) thin film memristive devices are fabricated by magnetron sputtering method. The influence of defect formation and carrier transport properties on memristive devices performances are characterized. The length of nanocrystal CeO2-x grain is about 8–12 nm and the diameter is about 6–8 nm with abundant grain boundaries (GBs). The memristive device exhibits an electronic conductivity. Under the forward bias, electrons accumulate in the vicinity of GBs due to space electrons zone effect and hop among oxygen vacancies to form the conductive filament (CF). The set voltage and reset voltage ranges from ~1.8 to 6 V and from ~-0.3 to -1 V, respectively. The cross-point memory shows an appropriate resistance ratio of high resistance state (HRS) to low resistance state (LRS) is about 104. The retention times of the devices can be more than 104 S, exhibiting a stable performance in terms of information storage capability.

Amplified Spontaneous Emission Threshold Reduction and Operational Stability Improvement in CsPbBr3 Nanocrystals Films by Hydrophobic Functionalization of the Substrate

The use of lead halide perovskites in optoelectronic and photonic devices is mainly limited by insufficient long-term stability of these materials. This issue is receiving growing attention, mainly owing to the operational stability improvement of lead halide perosvkites solar cells. On the contrary, fewer efforts are devoted to the stability improvement of light amplification and lasing. In this report we demonstrate that a simple hydrophobic functionalization of the substrates with hexamethyldisilazane (HMDS) allows to strongly improve the Amplified Spontaneous Emission (ASE) properties of drop cast CsPbBr3 nanocrystal (NC) thin films. In particular we observe an ASE threshold decrease down to 45% of the value without treatment, an optical gain increase of up to 1.5 times and an ASE operational stability increase of up to 14 times. These results are ascribed to a closer NC packing in the films on HMDS treated substrate, allowing an improved energy transfer towards the larger NCs within the NC ensemble, and to the reduction of the film interaction with moisture. Our results propose hydrophobic functionalization of the substrates as an easy approach to lower the ASE and lasing thresholds, while simultaneously increasing the active material stability.

Electrically Stimulated Synaptic Resistive Switch in Solution-Processed Silicon Nanocrystal Thin Film: Formation Mechanism of Oxygen Vacancy Filament for Synaptic Function

We demonstrate an electrically stimulated synaptic resistive switch in a silicon nanocrystal (Si NC) thin film. A forming-free resistive switching occurs on the surface of natively oxidized Si NCs because of filaments of oxygen vacancies. To show a gradual change of the resistance, we investigate a formation mechanism of an oxygen vacancy filament. This gradual change in the resistance with input voltage pulses corresponds to short-term plasticity (STP) and long-term potentiation (LTP) in biological synapses. We simulate spike-timing-dependent plasticity (STDP) in the resistive switch by voltage pulses.

Optical and surface properties of CdSe thin films prepared by sol-gel spin coating method

In this work, L-cysteine capped CdSe nanocrystals thin films were grown on glass substrate by using spin-coating method. The annealing temperature effect on structural, morphological, and optical properties of CdSe thin film is investigated by X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Atomic force microscopy (AFM), UV–Vis and photoluminescence (PL) techniques. XRD analysis showed crystallization and phase transformation of CdSe thin films from cubic (Zinc Blende, β-CdSe) to hexagonal (Wurtzite, α-CdSe) structure are obtained after air-annealing at 250 °C during 30 min. DSC analysis showed the exothermic peak in the range of 170–225 °C, indicating the structural phase transformation of CdSe thin film. AFM analysis exhibited a change in the surface roughness of thin films and the shape of CdSe nanoparticles from hemispherical nanocrystals into microrods as a function of annealing temperature. The surface and volume energy loss (VELF and SELF) functions show different dependences on the incident photon energy for the CdSe films in the energy range 1.5–4 eV as a function of air-annealing temperature. The dispersive optical parameters such as refractive index (n), extinction coefficient (k), and the dielectric constants were found to vary with the annealing temperature. PL measurements showed that the increase in the annealing temperature up to 200 °C results an improvement in the crystalline quality of the CdSe thin film, a decrease in their optical band gap, and an enhancement in the PL peak intensity.

Designing High-Performance CdSe Nanocrystal Thin-Film Transistors Based on Solution Process of Simultaneous Ligand Exchange, Trap Passivation, and Doping

We report a simple, solution-based, and postsynthetic process for simultaneous ligand exchange, surface passivation, and doping of CdSe nanocrystals (NCs) for the design of high-performance field-e...

Microstructure and Size Effects on the Mechanics of Two Dimensional, High Aspect Ratio Nanoparticle Assemblies

Uniaxially arranged nanocomposite structures are common across biological materials. Through efficient structural ordering and hierarchies, these materials exhibit stiffnesses and strengths comparable to the better of their constituents. While much is known regarding the mechanical properties of materials with explicit stiff and compliant phases, substantially less is understood about neat (matrix-free) materials composed from high aspect ratio particles. A promising example of nanoparticle assemblies are cellulose nanocrystal (CNC) thin films, which display desirable elastic and failure properties. Despite the exceptional properties of CNC films, accurately linking their nanoscopic properties to macroscale performance remains a challenge. Therefore, this work assesses the effects of fiber geometry and in-plane topology on the elastic and failure behaviors of neat CNC thin films using an atomistically informed coarse-grained molecular dynamics model. Short fiber films show a greater dependence on their specific in-plane ordering compared to longer fiber films. Furthermore, aligned, brick and mortar type CNC films exhibit a remarkable resiliency to random structural perturbations, particularly for films built from long fibers. Finally, simulation size is shown to affect the apparent failure properties of CNC films, with no meaningful impact on elastic property predictions. The relationships between structure and fiber length, as well as the sensitivities to structural randomness and simulation size, elucidated herein provide a comprehensive overview of the expected mechanics of high aspect ratio nanoparticle assemblies.

Chemical Effect of Halide Ligands on the Electromechanical Properties of Ag Nanocrystal Thin Films for Wearable Sensors

We investigated the chemical effects of halide ligands on the electromechanical properties of Ag nanocrystal (NC) thin films for potential uses in wearable devices and sensors. The halide treatments induced changes in the sizes of sintered NCs, interparticle distances, and microscale surface morphologies. Various characterization techniques and models were used to study the origin of nanoscale and microscale structures, their surface chemistries, and their effects on the electronic and electromechanical properties of the Ag NC thin films. The results indicated that the halide treatments led to changes in the electromechanical gauge factor, which varied from 5 to 600. On the basis of these controllable properties, stable wearable electrodes and sensitive gauge sensors were fabricated. Finally, through all-solution processing, we fabricated directly readable wearable circuits and highly sensitive sensors, in which the motion is detected by the intensity of light through the naked eye. We believe that this work will provide fundamental understanding of the chemical effects of nanostructures on electromechanical properties and a pathway to developing a low-cost, high-performance wearable technology.

Surface Versus Impurity-Doping Contributions in InAs Nanocrystal Field Effect Transistor Performance

The electrical functionality of an array of semiconductor nanocrystals depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs nanocrystals thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped nanocrystal based field-effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NCs based FETs. As-synthesized InAs nanocrystal-based FETs show n-type characteristics assigned to the contribution of surface electrons accumulation layer that can be considered as an actual electron donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NCs FETs show enhanced n-type conduction as expected from the Cu impurities location as an interstitial n-dopant in InAs nanocrystals. The simulated curves reveal the additional contribution from electrons within an impurity sub-band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC-based FETs. It provides guidelines for control of doping of nanocrystal arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs towards their future utilization as building blocks in bottom-up prepared optoelectronic devices.

Highly Sensitive Temperature Sensor: Ligand‐Treated Ag Nanocrystal Thin Films on PDMS with Thermal Expansion Strategy

AbstractHighly sensitive temperature sensors are designed by exploiting the interparticle distance–dependent transport mechanism in nanocrystal (NC) thin films based on a thermal expansion strategy. The effect of ligands on the electronic, thermal, mechanical, and charge transport properties of silver (Ag) NC thin films on thermal expandable substrates of poly(dimethylsiloxane) (PDMS) is investigated. While inorganic ligand‐treated Ag NC thin films exhibit a low temperature coefficient of resistance (TCR), organic ligand‐treated films exhibit extremely high TCR up to 0.5 K−1, which is the highest TCR exhibited among nanomaterial‐based temperature sensors to the best of the authors' knowledge. Structural and electronic characterizations, as well as finite element method simulation and transport modeling are conducted to determine the origin of this behavior. Finally, an all‐solution based fabrication process is established to build Ag NC‐based sensors and electrodes on PDMS to demonstrate their suitability as low‐cost, high‐performance attachable temperature sensors.

An evaluation of fluorinated titanium oxide nanocrystals with UV exposure treatment for oxygen vacancy control

Abstract The oxygen vacancy properties of densified titanium oxide (TiO2) nanocrystals (NCs) with different surface ligand states were evaluated. Fluorous ligand modification resulting in the high densification of NCs and the effect of residual ligands on the surface of the NCs using ultraviolet (UV)-exposure treatment were investigated. After synthesizing the TiO2 NCs at 80 °C, their modification proceeded using 2,2,2-trifluoroacetic acid as the fluorous ligand. NC thin films were formed using spin-casting, after which the UV-exposure treatment was performed. According to the analytical results of crystalline size, surface, optical properties, and surface ligand states of the TiO2 NCs, the ligands on the NCs were decomposed by the UV-exposure treatment under various atmospheres and the flatness of the NC thin films was found to be controlled by the fluorous ligand modification. Thus, the recombination effect of the oxygen vacancies could be controlled by the fluorous ligand modification and UV-exposure treatment.

Correction to “Investigation of the Chemical Effect of Solvent during Ligand Exchange on Nanocrystal Thin Films for Wearable Sensor Applications”

Correction to “Investigation of the Chemical Effect of Solvent during Ligand Exchange on Nanocrystal Thin Films for Wearable Sensor Applications”

Ultrafast THz Probe of Photoinduced Polarons in Lead-Halide Perovskites.

We study the nature of photoexcited charge carriers in CsPbBr_{3} nanocrystal thin films by ultrafast optical pump-THz probe spectroscopy. We observe a deviation from a pure Drude dispersion of the THz dielectric response that is ascribed to the polaronic nature of carriers; a transient blueshift of observed phonon frequencies is indicative of the coupling between photogenerated charges and stretching-bending modes of the deformed inorganic sublattice, as confirmed by DFT calculations.

Control of electrical conductivity of highly stacked zinc oxide nanocrystals by ultraviolet treatment

Zinc oxide (ZnO) nanocrystals (NCs) were synthesized using a modified sol-gel method. Ultraviolet (UV) treatment was performed under various atmospheres on the highly stacked ZnO NCs. The prepared NCs were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, X-ray diffraction, photoluminescence spectroscopy, and atomic force microscopy to investigate their structural, electrical, and electrochemical properties. Through these analyses, the effect of the UV treatment on the chemical and electrical characteristics of ZnO NCs was established. According to the analyses, the organic ligands in the NCs were decomposed, and the particles were densified. The mobility of UV-treated ZnO NCs thin films increased to 1.4 cm2/Vs, almost 2 orders higher than the UV untreated ZnO thin films. It was confirmed that the recombination from oxygen vacancies of ZnO could be controlled by UV irradiation. As decreased oxygen vacancies, the band gap of ZnO NCs was increased from 3.2 eV to 3.27 eV.

The Effects of ZnTe:Cu Back Contact on the Performance of CdTe Nanocrystal Solar Cells with Inverted Structure.

CdTe nanocrystal (NC) solar cells have received much attention in recent years due to their low cost and environmentally friendly fabrication process. Nowadays, the back contact is still the key issue for further improving device performance. It is well known that, in the case of CdTe thin-film solar cells prepared with the close-spaced sublimation (CSS) method, Cu-doped CdTe can drastically decrease the series resistance of CdTe solar cells and result in high device performance. However, there are still few reports on solution-processed CdTe NC solar cells with Cu-doped back contact. In this work, ZnTe:Cu or Cu:Au back contact layer (buffer layer) was deposited on the CdTe NC thin film by thermal evaporation and devices with inverted structure of ITO/ZnO/CdSe/CdTe/ZnTe:Cu (or Cu)/Au were fabricated and investigated. It was found that, comparing to an Au or Cu:Au device, the incorporation of ZnTe:Cu as a back contact layer can improve the open circuit voltage (Voc) and fill factor (FF) due to an optimized band alignment, which results in enhanced power conversion efficiency (PCE). By carefully optimizing the treatment of the ZnTe:Cu film (altering the film thickness and annealing temperature), an excellent PCE of 6.38% was obtained, which showed a 21.06% improvement compared with a device without ZnTe:Cu layer (with a device structure of ITO/ZnO/CdSe/CdTe/Au).