Large Bulk Crystals Research Articles

Intrinsic magnetic properties of the layered antiferromagnet CrSBr

van der Waals magnetic materials are an ideal platform to study low-dimensional magnetism. Opposed to other members of this family, the magnetic semiconductor CrSBr is highly resistant to degradation in air, which, in addition to its exceptional optical, electronic, and magnetic properties, is the reason the compound is receiving considerable attention at the moment. For many years, its magnetic phase diagram seemed to be well-understood. Recently, however, several groups observed a magnetic transition in magnetometry measurements at temperatures of around 40 K that is not expected from theoretical considerations, causing a debate about the intrinsic magnetic properties of the material. In this Letter, we report the absence of this particular transition in magnetization measurements conducted on high-quality CrSBr crystals, attesting to the extrinsic nature of the low-temperature magnetic phase observed in other works. Our magnetometry results obtained from large bulk crystals are in very good agreement with the magnetic phase diagram of CrSBr previously predicted by the mean-field theory; A-type antiferromagnetic order is the only phase observed below the Néel temperature at TN = 131 K. Moreover, numerical fits based on the Curie–Weiss law confirm that strong ferromagnetic correlations are present within individual layers even at temperatures much larger than TN.

Scintillation and radioluminescence mechanism in β-Ga2O3 semiconducting single crystals

In this paper, we present the results of experiments on samples of β-Ga2O3 single crystals under a project aimed at assessing and improving the scintillation performance of this material by studying scintillation and radioluminescence mechanism and its limitations. In addition to standard experiments, such as scintillation light yields and time profiles, radio-, and thermoluminescence, we developed and tested a new and promising two-beam experiment, in which a sample is excited by an X-ray beam and additionally stimulated by an IR laser diode.Fe and Mg doping compensate for the inherent n-type conductivity of β-Ga2O3 to obtain semi-insulating single crystals for large-area substrates and wafers. At the same time, residual Fe and Ir are ubiquitous uncontrolled impurities leached from the Ir crucibles used to grow large bulk crystals by the Czochralski method.For these experiments, we selected four samples cut from the Czochralski grown 2-cm diameter β-Ga2O3 single crystal boules; one with a reduced Fe content, two unintentionally Fe- and Ir-doped (UID) with lower and higher Fe content, and one doped with Mg.We find that steady-state radioluminescence spectra measured at temperatures between 10 and 350 K are dominated by the UV emission peaking at about 350–370 nm. Unfortunately, even for the best sample with a reduced Fe-content, the intensity of this emission drops precipitously with the temperature down to about 10 % at 300 K.From the two-beam experiments, we conclude that recombination via inadvertent Fe impurity involving three charge states (2+, 3+, and 4+) may reduce a steady-state UV emission of β-Ga2O3 under X-ray excitation by as much as 60–70 %, one-third to one-half of which is due to the recombination (specific for Fe-doped β-Ga2O3) involving the 4+ and 3+ charge states of Fe and the remaining 50–70 % being due to a more familiar route typical of other oxides, involving the 2+ and 3+ charge states of Fe. These losses are at higher temperatures enhanced by a thermally activated redistribution of self-trapped holes (STHs). In addition, the trapping of electrons by Fe and holes by Mg, Fe, and Ir may be responsible for scintillation light loss and reduction of the zero-time amplitude essential for the fast timing scintillation applications.Despite indirect evidence of competitive recombination in β-Ga2O3 involving a deep Ir3+/4+ donor level, we could not quantitatively assess losses of the UV steady state radioluminescence light due to the inadvertent Ir impurity.

Fabrication of a Field-Effect Transistor Based on 2D Novel Ternary Chalcogenide PdPS.

Two-dimensional (2D) palladium phosphide sulfide (PdPS) has garnered significant attention, owing to its exotic physical properties originating from the distinct Cairo pentagonal tiling topology. Nevertheless, the properties of PdPS remain unexplored, especially for electronic devices. In this study, we introduce the thickness-dependent electrical characteristics of PdPS flakes into fabricated field-effect transistors (FETs). The broad thickness variation of the PdPS flakes, ranging from 0.7-306 nm, is prepared by mechanical exfoliation, utilizing large bulk crystals synthesized via chemical vapor transport. We evaluate this variation and confirm a high electron mobility of 14.4 cm2 V-1 s-1 and Ion/Ioff > 107. Furthermore, the 6.8 nm-thick PdPS FET demonstrates a negligible Schottky barrier height at the gold electrode contact, as evidenced by the measurement of the temperature-dependent transfer characteristics. Consequently, we adjusted the Fowler-Nordheim tunneling mechanism to elucidate the charge-transport mechanism, revealing a modulated mobility variation from 14.4 to 41.2 cm2 V-1 s-1 with an increase in the drain voltage from 1 to 5 V. The present findings can broaden the understanding of the unique properties of PdPS, highlighting its potential as a 2D ternary chalcogenide in future electronic device applications.

Interplay between salt precipitation, corner liquid film flow, and gas–liquid displacement during evaporation in microfluidic pore networks

Visualization experiments with microfluidic pore networks are performed in this work to disclose interplay between salt precipitation, the corner liquid film flow, and gas–liquid displacement during evaporation. Two forms of salt precipitation are revealed: aggregated polycrystalline structures and large bulk crystals. It is found that gas bubbles can be formed because of imbibition of liquid into aggregated polycrystalline structures. The length of a corner liquid film can affect the direction of growth of the aggregated polycrystalline structures connected to the corner liquid film. Discontinuous corner liquid films can be transformed to continuous ones when they are touched by growing aggregated polycrystalline structures. The “sleeping” aggregated polycrystalline structures at the open surface of a microfluidic pore network, i.e., efflorescence, can grow again if they are touched by growing aggregated polycrystalline structures inside the microfluidic pore network, i.e., subflorescence. Because of efflorescence, the evaporation rate from a microfluidic pore network can increase first and then decrease. Moreover, a theoretical model is developed for the coupled transport of vapor diffusion in the gas zone and liquid flow as well as transport of dissolved salt in the corner liquid films in a capillary tube of square cross section so as to disclose the key parameters controlling the transport processes.

In Situ Preparation of High‐Quality Flexible Manganese‐Halide Scintillator Films for X‐Ray Imaging

AbstractEnvironmentally friendly metal halides have emerged as emitters in lighting and X‐ray imaging applications. However, the conventional scintillator fabrication process involves high‐temperature sintering or powder grinding, resulting in large bulk crystals or agglomerates, which hamper flexible device integration and processability. Here, large‐area flexible scintillation screen films containing (C24H20P)2MnBr4 nanocrystals are prepared in situ, which prevents the generation of agglomerated powders or large bulk crystals, and the preparation duration and cost are reduced. The film exhibits good mechanical stability and homogeneity, and the photoluminescence quantum yield is over 85%. Moreover, the film presents excellent scintillation performance with detection limit as low as 0.608 μGyair s−1 and 14.5 lp mm−1 X‐ray imaging resolution. This excellent performance and simple preparation method are expected to favor industrial mass production.

Two-Inch Aluminum Nitride (AIN) Single Crystal Growth for Commercial Applications

Aluminum Nitride (AlN) is an Ultra-wide Bandgap (UWBG) semiconductor that attracts significant interest for its potential application for high power switches, high power density, high frequency and high operating temperature for rf. Currently, AlN substrates are being used for ultraviolet (UV) light emitters at wavelengths shorter than 300nm including UVC Light Emitting Diodes (UVC LED) as well as laser diodes. Successful commercialization of the aluminum nitride as an UWBG substrate depends on several factors such as substrate availability, size, quality, and cost. Here we report on the current status of the bulk growth of AlN single-crystals as well as their subsequent processing for making substrates.The AlN crystals are grown using a proprietary physical vapor transport (PVT) technique where a sublimation-recondensation approach is utilized. This method shows very high potential in terms of growing large (> 2-in in diameter) bulk AlN crystals. However, some challenges have yet to be resolved en route to 100 mm diameter crystals. The crystal growth is carried out in a Tungsten crucible that contains the AlN seed and the source material. The AlN source material sublimes and decomposes into Aluminum atoms and Nitrogen molecules. These species are then transported to the seed where they form AlN and incorporate into the growing crystal along the c-axis. Aluminum nitride is used as a native seed utilizing the Al-polar face. Increasing the crystal dimensions demands careful tailoring of thermal gradients to lower the thermally induced stresses and achieve high crystalline quality. Currently, AlN single crystals with diameters > 2 inch are grown using the method described above and larger diameters are possible with appropriate furnace design. Large bulk crystals could be beneficial for production of non-polar and semi-polar substrates which are used to reduce the internal electric fields due to polarization.The AlN crystals are sliced into c-face oriented wafers and subsequently polished. Several substrate parameters are tracked such as the wafer bow, thickness variation, surface roughness, and sub-surface damage. The two-inch AlN wafers have a radius of curvature ≥ 20 m, RMS ≤ 5 Å, and zero sub-surface damage. The crystalline quality is measured by X-ray diffractometry (XRD) rocking curves, etch pit density, and photo-elastic birefringence pattern (cross-polar imaging). Narrow rocking curves for both symmetric and asymmetric reflections with full width at half maximum (FWHM) of 6 - 10 arcsec are measured across the whole the wafer area. The AlN wafers are transparent in the UV region. The graph shows a 52-point map of the absorption coefficient values 8 – 10 cm-1 at 265nm wavelength for a 2-in AlN substrate. The very low UV absorption is linked to the material purity measured by Secondary Ion Mass Spectrometry (SIMS) and Glow Discharge Mass Spectrometry (GDMS). Excellent material purity ensures achievement of high thermal conductivity, a crucial parameter for an UWBG substrate. We measure the thermal conductivity using a Xenon flash method conforming to the ASTM E1461-01 or ASTM E1461-13 standards. Using this method directly on 2-inch AlN substrate we obtain room temperature thermal conductivity 293 W/m*K at room temperature (RT) and with very high slope of about -1.6 for the temperature range from RT to 300°C suggesting minimal phonon scattering.AlN substrates made using the above described methods are used to fabricate high performance, long lifetime UVC LEDs for air purification, water disinfection, surface cleaning, and environmental sensing. High quality pseudomorphic epitaxial layers are grown using Metal-Organic Chemical Vapor Deposition (MOCVD) process to form the device structure. Commercially available WD product (Klaran™) achieve germicidal power outputs that exceed 80 mW with drive currents up to 700 mA at a nominal wavelength of 265 nm. The devices feature long lifetimes exceeding 6000 hours at L50B10 as well as a wide radiation pattern with a viewing angle of 130°. Figure 1

Preparation of Lead-free Two-Dimensional-Layered (C8H17NH3)2SnBr4 Perovskite Scintillators and Their Application in X-ray Imaging.

Scintillators, as spectral and energy transformers, are essential for X-ray imaging applications. However, their current disadvantages, including high-temperature sintering and generation of agglomerated powders or large bulk crystals, may not meet the increasing demands of low cost, nontoxicity, and flexible radiation detection. Thus, improved perovskite scintillators are developed in this research. A hybrid perovskite ((C8H17NH3)2SnBr4), which is nontoxic, lead-free, and organic-inorganic, is developed as a scintillator with good emission performance and radioluminescence intensity. These perovskite scintillators are synthesized at low temperatures in an aqueous acid solution, through which they generate a near-unity photoluminescence quantum yield of 98% with the excitation of ultraviolet light. As far as we know, this work is the first to show that the two-dimensional (2D) (C8H17NH3)2SnBr4 perovskite scintillator films prepared by coating a polymer layer can be applied to an X-ray imaging system. The results demonstrate that the low cost X-ray imaging device with good resolution and performance benefits dramatically from this lead-free organic-inorganic hybrid perovskite film. Therefore, this 2D-layered (C8H17NH3)2SnBr4 perovskite scintillator may be a high potential candidate for scintillating material for X-ray imaging techniques.

Metal Halide Perovskite Nanosheet for X-ray High-Resolution Scintillation Imaging Screens.

Scintillators, which are capable of converting ionizing radiation into visible photons, are an integral part of medical, security, and commercial diagnostic technologies such as X-ray imaging, nuclear cameras, and computed tomography. Conventional scintillator fabrication typically involves high-temperature sintering, generating agglomerated powders or large bulk crystals, which pose major challenges for device integration and processability. On the other hand, colloidal quantum dot scintillators cannot be cast into compact solid films with the necessary thickness required for most X-ray applications. Here, we report the room-temperature synthesis of a colloidal scintillator comprising CsPbBr3 nanosheets of large concentration (up to 150 mg/mL). The CsPbBr3 colloid exhibits a light yield (∼21000 photons/MeV) higher than that of the commercially available Ce:LuAG single-crystal scintillator (∼18000 photons/MeV). Scintillators based on these nanosheets display both strong radioluminescence (RL) and long-term stability under X-ray illumination. Importantly, the colloidal scintillator can be readily cast into a uniform crack-free large-area film (8.5 × 8.5 cm2 in area) with the requisite thickness for high-resolution X-ray imaging applications. We showcase prototype applications of these high-quality scintillating films as X-ray imaging screens for a cellphone panel and a standard central processing unit chip. Our radiography prototype combines large-area processability with high resolution and a strong penetration ability to sheath materials, such as resin and silicon. We reveal an energy transfer process inside those stacked nanosheet solids that is responsible for their superb scintillation performance. Our findings demonstrate a large-area solution-processed scintillator of stable and efficient RL as a promising approach for low-cost radiography and X-ray imaging applications.

Ultra-fast scintillation properties of [formula omitted]-Ga2O3 single crystals grown by Floating Zone method

In this investigation, β-Ga2O3 single crystals were grown by the Floating Zone method. At room temperature, the X-ray excited emission spectrum includes ultraviolet and blue emission bands. The scintillation light output is comparable to the commercial BGO scintillator. The scintillation decay times are composed of the dominant ultra-fast component of 0.368 ns and a small amount of slightly slow components of 8.2 and 182 ns. Such fast component is superior to most commercial inorganic scintillators. In contrast to most semiconductor crystals prepared by solution method such as ZnO, β-Ga2O3 single crystals can be grown by traditional melt-growth method. Thus we can easily obtain large bulk crystals and mass production.

Aquifer-on-a-Chip: understanding pore-scale salt precipitation dynamics during CO2 sequestration

In this study, we develop a lab-on-a-chip approach to study pore-scale salt precipitation dynamics during CO2 sequestration in saline aquifers-a challenge with this carbon management strategy. Three distinct phases-CO2 (gas), brine (liquid), and salt (solid)-are tracked through microfluidic networks matched to the native geological formations. The resulting salt formation dynamics indicate porosity decreases of ~20% in keeping with large scale core studies. At the network scale, the salt precipitation front moves at a constant velocity, ~2% that of the superficial CO2 velocity in this case. At the pore-scale, we observe two dominant types of salt formation: (1) large bulk crystals, on the order of the pore size (20-50 μm), forming early within trapped brine phases; and (2) polycrystalline aggregated structures, ranging over broad length scales, forming late in the evaporation process and collecting/projecting from the CO2-brine interface. Together, these two salt formation mechanisms show particular propensity for pore blockage and reduced carbon storage capacity.

Bulk growth of cadmium mercury telluride (CMT) using the Bridgman/accelerated crucible rotation technique (ACRT)

Cadmium mercury telluride (CMT, Cd x Hg 1− x Te) is still the pre-eminent infrared material, despite the difficulties associated with its production and subsequent processing. By varying the x value the material can be made to cover all the important infrared (IR) ranges of interest (i.e. 1–3, 3–5 and 8–14 μm). This paper will focus on the technological aspects of the production of large bulk crystals, their assessment and their use in photoconductive IR detectors. Bridgman growth takes place in simple two-zone furnaces with the pure elements contained in thick-walled high-purity silica ampoules. The thick ampoule walls are needed to contain the high (up to ∼70 atmos) mercury vapour pressures within the ampoules. We have found it necessary to purify both the mercury and the tellurium on site before use to obtain the required electrical properties. There is marked segregation of the matrix elements in standard Bridgman growth that is both a disadvantage and an advantage. Its disadvantage is that the yield of material in terms of composition for the most commonly required regions is low. The advantage is that all regions of interest can be produced in the same crystal, to some extent. A further advantage is that segregation of impurities also occurs and this leads to low background donor levels in Bridgman material. The other main disadvantages of the Bridgman technique are that the material is non-uniform in composition in the radial direction, as well as in the growth direction, resulting in low yields, and there are numerous grain and sub-grain boundaries. We have therefore developed an improved process based on the addition of the accelerated crucible rotation technique (ACRT). Here, ampoules are subjected to periodic acceleration/deceleration in their rotation, rather than constant rotation as in the Bridgman process. The major effect of this is to stir the melt during growth and produce flatter solid/liquid interfaces. This, in turn, improves the radial and axial compositional uniformity of the material, normally by a factor of at least ten-fold, and reduces somewhat the grain and sub-grain densities. The only drawback is that the long-wavelength material is now p-type as-grown and must be annealed in mercury vapour to convert it to n-type for use in photoconductive detectors, although the 1–3 and 3–5-μm material is n-type as-grown. An additional marked advantage of ACRT is that the improved radial compositional uniformity enables larger diameter material to be produced (up from 13 to 20 mm in our case). Assessment of the material includes wavelength mapping of both radially cut slices and axially cut planks; the latter gives useful information on the shape and change in the solid/liquid interface as growth proceeds. Hall-effect measurements are used routinely to characterise the material electrically. Images taken with an IR camera, and a blackbody source, reveal features in slices, e.g. cracks, inclusions of second phase and swirl patterns; the origin of the latter is as yet unknown. Some chemical analysis data from laser scan mass spectrometry (LSMS) for earlier 13-mm-diameter material and secondary ion mass spectrometry (SIMS) for later 20-mm-diameter material is also presented, which demonstrates the high-purity levels reached in this material. We are currently routinely growing 20-mm-diameter, 200-mm-long crystals of ∼0.5-kg weight with good uniformity of composition and good electrical properties for first-generation photoconductive IR detector programmes that require low carrier concentration n-type material. Compositions produced range from x∼0.7 (∼1.3 μm) through to x∼0.0 (i.e. HgTe), the latter in the tail ends of the crystals.

In-situ observations of the behaviors of ferroelectric domains in BaTiO3 under applied electric fields with TEM

The previous studies on the behaviors of ferroelectric domains under applied electric fields were made by a number of researchers, e.g. Merz and Little on bulk BaTiO3 crystals with optical microscopy. It was suggested that under the applied electric field the new antiparallel domains nucleated and grew and 90° and 180° domains nucleated and grew sidewise. However, those results and conclusions were obtained from the experiments on large bulk crystals with optical microscopy of relatively lower magnification and resolution. TEM is a very powerful tool in the study of crystal structure and defects and may provide a new interpretation to the study of microdomains of hundred angstroms in thin ferroelectric films with a higher magnification and resolution and may be combined with electron diffraction. However, no reference has been found that an in-situ TEM study of the behaviors of ferroelectric materials under applied electric fields has ever been made.