Large-area Crystals Research Articles

Moisture-dependent room-temperature perovskite crystallization in vacuum flash-assisted solution processed intermediate phase films

We propose a straightforward intermediate-phase strategy to facilitate large-area crystallization of perovskite under room-temperature: preparing intermediate phase film with vacuum flash-assisted solution processing (VASP) method and performing controlled moisture-exposure subsequently. Moisture molecules penetrate into the whole intermediate film, remove residual solvent molecules, and trigger room-temperature crystallization of perovskite in a wide humidity range (25–80% RH), while negligible hydroxyl species are observed in the intermediate phase and resultant perovskite films. Without antisolvent, thermal-annealing and additive, high-humidity (80% RH) exposure of intermediate-phase film under room-temperature results in crystalline perovskite with a 8× increase in grain size and a 6× increase in photoluminescence lifetime, which is attributed to the homogenous and stable intermediate phase prepared by VASP and the synergistic effect of H 2 O and residual solvent molecules. Without antisolvent, thermal-annealing and additive, high-humidity (80% RH) exposure of intermediate-phase precursor film under room-temperature results in crystalline perovskite with a 8× increase in grain size and a 6× increase in photoluminescence lifetime. • The intermediate phase strategy applies in the wide range of 25–80% RH humidity. • Moisture exposure under room-temperature induces identical chemical transition. • Negligible hydroxyl species are observed in perovskite and its intermediate. • 80% RH exposure leads micron-size grains and microsecond fluorescence lifetime.

Effect of doping and annealing on resistivity, mobility-lifetime product, and detector response of (Cd,Mn)Te

We present the effect of doping with In, V, and Cl on the values of resistivity (ρ) and mobility-lifetime (μτ) in (Cd,Mn)Te crystals grown by the Bridgman method and with diameters of 2 or 3 in. This material is investigated to use in room-temperature X-ray and gamma-ray detectors. We carried out post-growth annealing processes in cadmium vapors at 500 °C to obtain more homogeneous crystal properties. The EU-ρ-μτ-SCAN machine, which uses the principle of time-dependent charge measurement, allowed us to map the values of resistivity and mobility-lifetime in crystal plates. High values of those two quantities are crucial in nuclear detector applications. Crystals doped with In show inhomogeneous properties, which makes the fabrication of competitive large-area crystals more difficult. Low-temperature post-growth annealing in Cd vapors is insufficient for (Cd,Mn)Te:In crystals, where the decrease of ρ and μτ values is observed. Such annealing works for (Cd,Mn)Te:V crystals, where the mean values of ρ and μτ do not change significantly, but their distributions are more uniform. Both dopants, vanadium and indium, used together with an excess of Te, effectively increased ρ and μτ. Using the 57Co source, the spectroscopic performance of each pixel of the detector co-doped with In and V was shown.

Stacking-dependent exciton multiplicity in WSe2 bilayers

Twisted layers of atomically thin two-dimensional materials realize a broad range of quantum materials with engineered optical and transport phenomena arising from spin and valley degrees of freedom and strong electron correlations in hybridized interlayer bands. Here, we report on experimental and theoretical studies of ${\mathrm{WSe}}_{2}$ homobilayers obtained in two stable configurations of $2H$ (${60}^{\ensuremath{\circ}}$ twist) and $3R$ (${0}^{\ensuremath{\circ}}$ twist) stackings by controlled chemical vapor synthesis of high-quality large-area crystals. Using optical absorption and photoluminescence (PL) spectroscopy at cryogenic temperatures, we uncover marked differences in the optical characteristics of $2H$ and $3R$ bilayer ${\mathrm{WSe}}_{2}$ which we explain on the basis of beyond-density functional theory calculations. Our results highlight the role of layer stacking for the spectral multiplicity of momentum-direct intralayer exciton transitions in absorption and relate the multiplicity of phonon sidebands in the PL to momentum-indirect excitons with different spin valley and layer character. Our comprehensive study generalizes to other layered homobilayer and heterobilayer semiconductor systems and highlights the role of crystal symmetry and stacking for interlayer hybrid states.

Crystallization of charged gold particles mediated by nonadsorbing like-charged polyelectrolyte.

We report that the aqueous dispersions of negatively charged submicron-sized colloidal Au particles formed non-close-packed colloidal crystals by the addition of a like-charged linear polyelectrolyte, sodium polyacrylate (NaPAA). Au particles often form irregular aggregates in dispersions because of a strong van der Waals force acting between them. To prevent aggregation, we introduced negative electric charges on particle surfaces. By the addition of NaPAA, colloidal crystals were formed on the bottom of a sample cell because of the supply of Au particles by sedimentation and 2D diffusion even under very dilute conditions. Interparticle potential calculations demonstrated that the addition of NaPAA caused depletion attraction between the particles as well as a significant reduction in the interparticle repulsion because of the electrostatic screening effect. However, the electrostatic repulsion was strong enough to prevent the direct contact of particles in the excluded region between Au particles. Large-area crystals could be obtained by tilting the sample cell. By drying the sample, the Au particles came into contact and the non-space-filling crystals changed into closest packed crystals. These closest packed crystals exhibited a significant enhancement of Raman scattering intensity because of high hot-spot density.

Effects of Solution Flow on the Growth of Colloidal Crystals.

For the versatile potential applications of colloidal crystals, precisely controlling their growth is required to achieve properties such as high crystallinity and large-area crystals. Because colloidal crystallization is a self-assembly process of dispersed particles in a solution, solution flow directly and markedly changes the behavior of particles. Thus, the effects of solution flow on the growth of colloidal crystals were investigated in the present study. We found three different effects of solution flow on the growth of colloidal crystals: enlarging the first layer, facilitating the growth of superlattice structures, and forming a new circular packing structure. Specifically, in the single-component system, because the flow speed is lower closer to the bottom of the cell, the second and further layers dissolve owing to the large flow speed, whereas the first layer remains undissolved at the appropriate flow speed. The dissolved particles (particles that are detached from the crystals and returned back into the aqueous medium) are transported near the first layer, where they facilitate the growth of the first layer. In a binary system, when colloidal crystals with different particles are neighboring each other, the flow dissolves the surface of each crystal, which forms a dense, melt-like phase between crystals, from which a superlattice structure such as AB2 grows. The flow often moves the second layer more than the first layer because the flow speed varies with the distance from the bottom. This causes the second layer to slide above the first layer of the neighboring crystals composed of different particle sizes, which transform from the initial face-centered cubic structure of the first layer into a circular pattern with strain. These findings contribute to establishing a sophisticated control method for growing colloidal crystals.

Study of Metal-Semiconductor-Metal CH3NH3PbBr3 Perovskite Photodetectors Prepared by Inverse Temperature Crystallization Method.

Numerous studies have addressed the use of perovskite materials for fabricating a wide range of optoelectronic devices. This study employs the deposition of an electron transport layer of C60 and an Ag electrode on CH3NH3PbBr3 perovskite crystals to complete a photodetector structure, which exhibits a metal–semiconductor–metal (MSM) type structure. First, CH3NH3PbBr3 perovskite crystals were grown by inverse temperature crystallization (ITC) in a pre-heated circulator oven. This oven was able to supply uniform heat for facilitating the growth of high-quality and large-area crystals. Second, the different growth temperatures for CH3NH3PbBr3 perovskite crystals were investigated. The electrical, optical, and morphological characteristics of the perovskite crystals were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet-visible spectroscopy, and photoluminescence (PL). Finally, the CH3NH3PbBr3 perovskite crystals were observed to form a contact with the Ag/C60 as the photodetector, which revealed a responsivity of 24.5 A/W.

Preliminary Analysis of Crystallization of Na2SO4 Solution in Silicate Cultural Relics

ABSTRACT Currently, there is a lack of research on the migration and distribution of soluble salt solutions in the pores of fragile silicate cultural relics, especially the thermodynamic changes during crystallization, and the forces causing the destruction of cultural relics. Thus, this paper used infrared thermal imaging analysis and extended depth-of-field microscopy to observe the changes in temperature, morphology, migration, and crystallization rate of Na2SO4 solution – the most typical solution that causes damages to cultural relics – in the crystallization process and in different capillary pores. The results show that the formation of soluble salt crystal nuclei and subsequent crystal growth mainly occurred in the liquid phase region below the ambient temperature, and then the salt crystals in the deliquescent state became more compact due to the attractive force generated from liquid-phase water evaporation. Additionally, three stages of migration and crystallization of the salt solution in different capillary pores were found. Large-area crystals appeared in the capillary pore with a large diameter, and bubbles were generated. This is likely because the surface tension of the Na2SO4 solution decreased with increasing concentration, and the solubility of the air dissolved at the solid–liquid interface of the mouth of the capillary pore and the pressure in the capillary pore decreased. A self-designed double-layer contact surface internal pressure method and extended depth-of-field microscopy were used to carry out real-time online observation of the force applied on the contact surface during the crystallization of the salt solution. It was found that the airflow determined the direction of capillary movement of the solution, and water migration determined the laminar flow in the capillary pore. Salt crystallization played a role in distortion and shape-fixation of pores. Small spikes in the mass indicated that the expansion–contraction force generated by the salt solution during the crystallization process was in a state of constant change, and the degree of expansion was always slightly more significant than the change caused by contraction. Expansion forces dominated the surface of the sample. In this paper, the change in crystallization and the forces of the soluble salt solution were analyzed.

Effect of the Air/Water Interfacial Properties of Amine Derivatives on the in Situ Fabrication of Microsized Gold Sheets.

Development of new methods for producing large-area nanocrystals with specific shapes is crucial for advancements in various fields. In this study, submillimeter-sized sheet-structured gold crystals with nanoscale thicknesses were fabricated by chemical reduction of HAuCl4 in the presence of long-chain amidoamine-derived surfactants (C nAOH; n = 12, 14, 16, or 18) in aqueous solutions. Using the C18AOH system at 30 °C, large-area sheet-structured crystals with widths of ∼100 μm and thicknesses of 30 nm were effectively obtained at the air/water interface. The crystal size depended on the temperature and the alkyl-chain length of the surfactant. An investigation of the relationship between the crystal growth and the interfacial properties of C nAOH revealed that large-area crystals were obtained when densely packed molecular layers of long-chain C nAOH were formed at the air/water interface. The interfacial molecular layer of C18AOH showed most effective soft-templating effect and contributed in promoting the growth of sheet-structured gold crystals.

Sublimation growth of bulk 3C-SiC using 3C-SiC-on-Si (1 0 0) seeding layers

We have developed a transfer process of 3C-SiC-on-Si (100) seeding layers grown by chemical vapor deposition onto a poly- or single-crystalline SiC carrier. Applying subsequent sublimation growth of SiC in [100] direction resulting in large area crystals (up to ≈11cm2) with a thickness of up to approximately 850μm. Raman spectroscopy, Laue X-ray diffraction and electron-backscattering-diffraction revealed a high material quality in terms of single-crystallinity without secondary polytype inclusions, antiphase boundaries or double positioning grain boundaries. Defects in the bulk grown 3C-SiC, like protrusions with surrounding stressed areas, stem from the epitaxial seeding layer. The presented concept using 3C-SiC-on-Si seeding layers reveals a path for the growth of bulk 3C-SiC crystals.

Microsecond Crystallization of Amorphous Silicon Films on Glass Substrates by Joule Heating

Large area crystallization of amorphous silicon (a-Si) thin-films on glass substrates is one of the key technologies for manufacturing flat displays on commercial scales. Low-temperature polycrystalline silicon (LTPS) is a crucial active-layer material for polycrystalline silicon thin-film-transistors (poly-Si TFTs) mounted on thermally susceptible glass substrates. LTPS-TFT backplanes are required for fabricating flat panel displays such as active-matrix liquid-crystal-displays and active-matrix organic-light-emitting diodes (AMOLEDs). The LTPS-TFT backplanes are strongly preferred over a-Si TFTs since several devices, AMOLEDs in particular, operate in a current-driven mode. The crystallization methods used for producing LTPS include solid phase crystallization, metal induced crystallization, and excimer laser crystallization (ELC). The ELC method is currently used for crystallizing the a-Si films in mass production lines. Recently, we developed a novel and highly promising crystallization method named Joule-heating induced crystallization (JIC) by which an entire a-Si film could be crystallized on a glass backplane within tens of microseconds. However, arc instability was observed during the crystallization, which is attributed to the dielectric breakdown in the conductor/insulator/a-Si sandwich-structures at high temperatures during electrical pulsing for crystallization. We devised a method for crystallizing the a-Si films while preventing arc generation; in this method an a-Si active layer was pre-patterned into islands followed by the deposition of a gate oxide and gate electrode. An electric pulse was then applied to the gate electrode formed using a Mo layer. The Mo-layer was used as a Joule-heat source for the crystallization of the pre-patterned active islands of the a-Si films located below the Mo-layer. The JIC-processed poly-Si TFTs were fabricated successfully and the proposed method was found to be compatible with the standard processing of coplanar top-gate poly-Si TFTs. In this report, we will discuss some critical issues concerning the arc generation, glass backplane deformation, and phase-transformation phenomena observed during the JIC process on the basis of theoretical investigations as well as in-situ and ex-situ experiments. Finally, we will discuss the procedure for fabricating stable JIC poly-Si TFTs and optimize the process flow and processing conditions such as the power density and pulsing time for fabricating the JIC poly-Si TFTS. Figure 1

2D materials and van der Waals heterostructures.

The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With these new materials, truly 2D physics has begun to appear (for instance, the absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc.). Novel heterostructure devices--such as tunneling transistors, resonant tunneling diodes, and light-emitting diodes--are also starting to emerge. Composed from individual 2D crystals, such devices use the properties of those materials to create functionalities that are not accessible in other heterostructures. Here we review the properties of novel 2D crystals and examine how their properties are used in new heterostructure devices.

High Efficiency Millimeter-Scale Crystalline Perovskite Solar Cells

Current state-of-the-art photovoltaics utilize high purity single-crystal semiconductors grown by sophisticated, high temperature crystal-growth processes. Alternative emerging solar cell technologies based on solution-processed materials promise to deliver power at lower cost, but so far have had limited success due to the presence of defects in the bulk and at the grain boundaries. Improving the crystal size and quality of solution-processed films could lead to unprecedented improvements in photovoltaic performance. However, large-scale crystal growth is challenging in solution-based deposition where molecular kinetics are limited by the low processing temperature. Here, for the first time we demonstrate organometallic perovskites thin-films composed of extra-ordinarily large crystalline grains (1-2 mm), fabricated using a new processing technique. The increase in the grain-size directly correlates to a dramatic increase in the efficiency, as a direct consequence of increased charge carrier mobility and reduction in defect densities. Our measured efficiency values approach ~18%, which is among the highest reported. These devices are extremely robust, exhibiting no degradation with voltage sweep direction or the rate at which the voltage was scanned. We anticipate that this technique for growing high quality large-area crystals at low temperature with low defect concentration will be universally applicable to a wide variety of other material systems thus providing a solution to a long-standing scientific challenge of overcoming polydispersity, defects and grain boundary recombination in solution-processed thin-films. From the perspective of the global photovoltaics community, these results are expected to lead the field towards the synthesis of wafer-scale crystalline perovskites necessary for the fabrication of high-efficiency single-junction and hybrid (semiconductor and perovskite) tandem planar cells. In addition, I will also discuss design principles that govern the photostability in these systems.

Large area crystallization of amorphous Si with overlapping high repetition rate laser pulses

This paper presents a pulsed laser crystallization technique, enabling large area crystallization of amorphous Si to produce grains having well-defined size and orientation. The method is developed by first determining the parameters influencing crystallization induced by single laser pulses of circular cross-sectional profile. In a second step, crystallization by overlapping round spots is examined. The experiments reveal three zones characterized by distinctly different crystallized morphologies following the laser irradiation. One of these zones corresponds to the regime of lateral crystal growth, wherein grains are driven towards the center of the spot by the radial temperature gradient. These findings are then applied to processing via line beam profiles that facilitate large area crystallization upon rapid translation of the specimen. Crystallization of extended areas hinges on the determination of the crystal growth length for a single spot. The pitch between successive pulses is then set on the basis of this information. It is shown that the pulse energy has only a weak effect on the crystal growth length.

The use of ATR-FTIR imaging to study coated oil capsules

Fourier transform infrared (FTIR) spectroscopic imaging using a large Internal Reflection Element (IRE), also called Attenuated Total Reflection (ATR) crystal, can be used for the identification and localisation of chemical compounds in solid food products. In this study ATR-FTIR imaging, by means of a large area crystal with a contact diameter of 600μm with a linear array detector, was used to elucidate the microstructure of the coating of a sophisticated multi-layered oil capsule with active ingredients (sodium ascorbate, sucrose, gelatine, gum, isoflavone). ATR-FTIR imaging with the aid of principal component analysis (PCA) showed the spatial location of the individual ingredients within the layers and that these ingredients are not uniformly distributed within the layers. By ATR-FTIR imaging it was determined that the layer thickness of the soft central coating layer containing lycopene, sodium ascorbate, sucrose and air is about ∼156μm, the 1st sucrose layer ∼180μm and the gelatine/gum ∼28μm. The latter was confirmed by X-ray microtomography (μCT). The other layers could not be identified by μCT.

비정질 실리콘의 결정화를 위한 줄 가열 유도 결정화 공정에 대한 열적 연구

대면적 비정질 실리콘 박막의 결정화는 평판 디스플레이 생산에 있어서 핵심 요소로 꼽힌다. 현재 다양한 결정화 기술들이 연구 되고 있으며 그 중 최근에 소개된 줄 가열 유도 결정화는 수십 마이크로초의 짧은 공정 시간, 대면적 결정화 그리고 국부적인 가열로 기판의 열변형 억제 등의 잇점으로 인해 AMOLED 제작에 있어서 기대되는 기술이다. 본 연구에서는 JIC 공정 중 상변화과정에서의 온도를 이론적으로 해석하고 이를 실험과 비교하였다. 이를 통하여 결정화 메커니즘을 결정하는 임계온도를 in-situ 실험과 수치해석을 통해 밝혀내었다.

Thermal deformation of glass backplanes during Joule-heating induced crystallization process

Large area crystallization of amorphous silicon (a-Si) thin-films on glass backplanes is one of key technologies in manufacturing flat displays of commercial scales. Among various crystallization technologies, Joule-heating induced crystallization (JIC) has been recently introduced and considered as the highly promising one since the whole film of a-Si on glass backplane can be crystallized within tens of microseconds. Based on theoretical investigation accompanied by in-situ and ex-situ experimentations, this study clarified the critical mechanism of the glass backplane deformation during the JIC process. Simulation using a commercial FEM code with one-dimensional conduction and viscoelastic models showed that the local region of which temperature was larger than the glass softening point undergoes structural shrinkage due to stress relaxation and this shrinkage remained permanently even after the process. As a quantification factor for such plastic deformation, the radius of curvature was estimated and compared to the experimental observation. Consequently the pulse duration should be decreased for larger display applications, because the radius of curvature could be considerably increased as the pulse duration was decreased for the same peak temperature of the JIC processes.

OLED 디스플레이 제작을 위한 Joule 유도 결정화 공정에서의 유리기판에 대한 열해석

Large area crystallization of amorphous silicon thin-films on glass substrates is one of key technologies in manufacturing flat displays. Among various crystallization technologies, the Joule induced crystallization (JIC) is considered as the highly promising one in the OLED fabrication industries, since the amorphous silicon films on the glass can be crystallized within tens of microseconds, minimizing the thermally and structurally harmful influence on the glass. In the JIC process the metallic layers can be utilized to heat up the amorphous silicon thin films beyond the melting temperatures of silicon and can be fabricated as electrodes in OLED devices during the subsequent processes. This numerical study investigates the heating mechanisms during the JIC process and estimates the deformation of the glass substrate. Based on the thermal analysis, we can understand the temporal and spatial temperature fields of the backplane and its warping phenomena.

Low-level gamma-ray spectrometry using Ge-detectors

The technological advances that have taken place during the past 45 years in the fabrication of high purity germanium crystals have had a significant impact on low-level gamma-ray spectrometry. Today, Ge-detectors with high purity Ge-crystals of 2 kg are readily available and detectors with crystals up to 5 kg can be produced. Use of large area crystals results in high detection efficiencies, which can serve to reduce detection limits or to make faster measurements of low activity samples possible. The second major development that has contributed to lowering detection limits has been research on identifying and reducing the background components in gamma-ray spectrometry systems. This paper deals with the major technical factors affecting the performance of low-background HPGe-detectors and provides an overview of the major areas of application.

Shear-ordered face-centred cubic photonic crystals

Large-area face-centred cubic photonic crystals are fabricated by the two-dimensional shear-alignment of stabilised 720 nm diameter PMMA spheres dispersed in an epoxy resin. The resulting crystal is made permanent by exposure to UV light, thus solidifying the epoxy resin. The technique is rapid and produces high quality, large-area crystals.

Evaluation of moderately cooled pure NaI as a scintillator for position-sensitive PET detectors

A new evaluation of pure NaI has been performed to determine if moderate cooling would lead to better performance than that of existing, activated NaI(Tl) position-sensitive detectors, particularly at high countrates. Using a freezer, an initial effort was performed to cool the crystal assembly to -90/spl deg/C (183/spl deg/K). At this temperature, pure NaI has a decay constant of 35 nsec, a light output which is about 20% that of room temperature NaI(Tl), and an energy resolution of 15%. For the authors' PET applications the signal of room temperature (25/spl deg/C) NaI(Tl) is normally pulse clipped, reducing the light output to 40% of the unclipped signal and yielding an energy resolution of 10.5%. Since the long decay of NaI(Tl) causes it to suffer more significantly than pure NaI from pre-pulse pileup, the difference in energy resolution between the two crystals at high countrates will be reduced. Also, a significantly shorter trigger deadtime with pure NaI will lead to a reduction in coincidence deadtime losses in PET. Computer simulations of large-area crystals operating at high countrates have been performed to quantify their trigger deadtime behavior and position resolution as a function of light output and pulse decay time. Having gained experience with the practical issues of cooling large crystals, measurements of position resolution have been performed with a NaI bar detector of similar geometry to the NaI(Tl) detectors in use in the PENN-PET scanner.