Engineering topics
Pan, Lei
Publications and source records attributed to Pan, Lei.
Investigation of leaching of nickel sulfide flotation tailings to recover valuable metals
Not provided.
Quantum Efficiency Gain in 2D Perovskite Photo and X‐Ray Detectors
Abstract The perovskite polycrystalline thin film detector fabricated by solution method is a promising low‐cost, scalable technology for radiation imaging, but its thin volume limits the sensing efficiency for high‐energy X‐ray photons. This work reports 2D perovskite thin film photo‐diodes with a detection gain when sensing visible and X‐ray photons. Detailed power and temperature‐dependent device characterizations reveal a charge multiplication effect to be responsible for the observed high gain. This is caused by a disparity in electron and hole transport, where electron transport is retarded by a trap/de‐trap process via shallow trap states, whereas the hole transport is fast enough to produce a photoconductive gain in satisfying the charge neutrality. The 2D perovskite made with butylamine spacers is also found to exhibit a larger efficiency gain than those made with phenylethylamines because of the higher likelihood of forming shallow traps in the former. The thin film diodes feature a high temporal response over 1 MHz due to the fast charge collection across a thin volume, and the discovery provides device physics mechanisms in connection to material structure‐function relationship for future optoelectronics design that can boost the efficiency of X‐ray sensing and dim light detection.
Low-damage electron beam lithography for nanostructures on Bi 2 Te 3 -class topological insulator thin films
Nanostructured topological insulators (TIs) have the potential to impact a wide array of condensed matter physics topics, ranging from Majorana physics to spintronics. However, the most common TI materials, the Bi 2 Se 3 family, are easily damaged during nanofabrication of devices. In this paper, we show that electron beam lithography performed with a 30 or 50 kV accelerating voltage—common for nanopatterning in academic facilities—damages both nonmagnetic TIs and their magnetically doped counterparts at unacceptable levels. We additionally demonstrate that electron beam lithography with a 10 kV accelerating voltage produces minimal damage detectable through low-temperature electronic transport. In conclusion, although reduced accelerating voltages present challenges in creating fine features, we show that with careful choice of processing parameters, particularly the resist, 100 nm features are reliably achievable.
Extreme γ –Ray Radiation Tolerance of Spectrometer–Grade CsPbBr 3 Perovskite Detectors
The perovskite compound CsPbBr 3 has recently been discovered as a promising room-temperature semiconductor radiation detector, offering an inexpensive and easy-to-manufacture alternative to the current benchmark material Cd 1–x Zn x Te (CZT). The performance of CsPbBr 3 sensors is evaluated under harsh conditions, such as high radiation doses often found in industrial settings and extreme radiation in space. Results show minimal degradation in detector performance after exposure to 1 Mrad of Co-60 gamma radiation, with no significant change to energy resolution or hole mobility and lifetime. Additionally, many of the devices are still functional after being exposed to a 10 Mrad dose over 3 days, and those that do not survive can still be refabricated into working detectors. Notably, these results suggest that the failure mode in these devices is likely related to the interface between the electrode and material and their reaction, or the electrode itself and not the material itself. Overall, the study suggests that CsPbBr 3 has high potential as a reliable and efficient radiation detector in various applications, including those involving extreme fluxes and energies of gamma-ray radiation.
Exchange-Biased Quantum Anomalous Hall Effect
The quantum anomalous Hall (QAH) effect is characterized by a dissipationless chiral edge state with a quantized Hall resistance at zero magnetic field. Manipulating the QAH state is of great importance in both the understanding of topological quantum physics and the implementation of dissipationless electronics. Here, the QAH effect is realized in the magnetic topological insulator Cr-doped (Bi,Sb) 2 Te 3 (CBST) grown on an uncompensated antiferromagnetic insulator Al-doped Cr 2 O 3 . Through polarized neutron reflectometry (PNR), a strong exchange coupling is found between CBST and Al-Cr 2 O 3 surface spins fixing interfacial magnetic moments perpendicular to the film plane. Further, the interfacial coupling results in an exchange-biased QAH effect. This study further demonstrates that the magnitude and sign of the exchange bias can be effectively controlled using a field training process to set the magnetization of the Al-Cr 2 O 3 layer. It demonstrates the use of the exchange bias effect to effectively manipulate the QAH state, opening new possibilities in QAH-based spintronics.
Study of perovskite CsPbBr 3 detector polarization and its mitigation with ultrahigh x-ray flux
High-flux capable semiconductor x-ray detectors are essential in various applications, but the detrimental effects of detector polarization limit their use in many cases. Here, for this work, we studied the polarization of perovskite CsPbBr 3 semiconductor detectors using ultrahigh flux synchrotron x rays (10 6 –10 12 photons s –1 mm –2 at 58.61 keV). The CsPbBr 3 detectors did not show immediate polarization prominently until a flux higher than 10 10 photons s –1 mm –2 . Using the pump-and-probe technique, we visualized the spatial and temporal effects of polarization. The polarized region, represented by reduced photocurrent, extended beyond the area under direct irradiation, and the reduced photocurrent persisted after potential de-polarization treatments. We found that stronger applied electric fields and fewer carrier traps can mitigate polarization, represented by less photocurrent deficit. By examining the detectors’ current response under controlled ambient light, low, and high-flux x rays, we studied the trap filling and release behavior of CsPbBr 3 . We discovered that the polarization is caused by partial detector damage due to deep defects generated by ultrahigh flux x-ray irradiation. Our work provides insight into CsPbBr 3 polarization under extremely intense x-ray radiation and shows that reducing crystal defects and increasing detector bias voltage are effective solutions.
Ultrahigh‐Flux X‐ray Detection by a Solution‐Grown Perovskite CsPbBr 3 Single‐Crystal Semiconductor Detector
Abstract Solution‐processed perovskites are promising for hard X‐ray and gamma‐ray detection, but there are limited reports on their performance under extremely intense X‐rays. Here, a solution‐grown all‐inorganic perovskite CsPbBr 3 single‐crystal semiconductor detector capable of operating at ultrahigh X‐ray flux of 10 10 photons s −1 mm −2 is reported. High‐quality solution‐grown CsPbBr 3 single crystals are fabricated into detectors with a Schottky diode structure of eutectic gallium indium/CsPbBr 3 /Au. A high reverse‐bias voltage of 1000 V (435 V mm − 1 ) can be applied with a small and stable dark current of ≈60–70 nA (≈9–10 nA mm − 2 ), which enables a high sensitivity larger than 10 000 µC Gy air −1 cm − 2 and a simultaneous low detection limit of 22 nGy air s − 1 . The CsPbBr 3 semiconductor detector shows an excellent photocurrent linearity and reproducibility under 58.61 keV synchrotron X‐rays with flux from 10 6 to 10 10 photons s − 1 mm − 2 . Defect characterization by thermally stimulated current spectroscopy shows a similar low defect density of a synchrotron X‐ray and a lab X‐ray irradiated device. Solid‐state nuclear magnetic resonance spectroscopy suggests that the excellent performance of the solution‐grown CsPbBr 3 single crystal may be associated with its good short‐range order, comparable to the spectrometer‐grade melt‐grown CsPbBr 3 .
Investigation of defects in melt and solution grown perovskite CsPbBr 3 single crystals
Recently, there has been considerable interest in x-ray and gamma ray detectors with large volume and high energy resolution that operate at room temperature. To improve detector energy resolution, the carrier mobility-lifetime product needs to be increased, and the electronic trap state concentration needs to be minimized. Defect concentrations in the part per billion range can alter the charge transport and carrier recombination lifetime. In this work, thermally stimulated current spectroscopy measurements were systematically carried out in bulk halide perovskite single crystals of CsPbBr 3 over a temperature range of 80–320 K. The origins and trap parameters of CsPbBr 3 crystals from the solution growth and melt growth procedures were determined and compared. Trap concentrations were ranged from 1 × 10 11 to 1 × 10 16 cm –3 . Appreciable detector performance was observed for CsPbBr3 crystals with trap concentrations less than 1 × 10 14 cm –3 . The comparison of spectral responses of crystal samples grown using two different methods shows that, after purification, solution-grown crystals are comparable to melt-grown crystals in terms of low defect concentration and improved detector performance. Finally, for an improved mobility-lifetime product and enhanced spectral response to high energy radiation from fissile materials, trap states in either type of a crystal ingot must be reduced closer to 10 11 cm –3 .
Laser Scribing for Electrode Patterning of Perovskite Spectrometer-Grade CsPbBr 3 Gamma-ray Detectors
Making semiconductor radiation detectors that work at room temperature relies heavily on the deposition and pixelation of electrodes. Electrode patterning of perovskite solar cells widely implements laser scribing techniques, which is a convenient, scalable, and inexpensive technique. However, this method has not found its application in radiation detector patterning yet, and the question whether laser scribing can achieve high-quality patterns with minimum damage to a detector crystal and low interpixel cross-talk remains largely unanswered. To prove that laser scribing is a practical method for electrode patterning on perovskite CsPbBr 3 detectors, we use the material to create a variety of patterns. A very low lateral leakage current (60 nA at 10 V) and high mobility-lifetime product (9.7(3) x 10 -4 cm 2 /V) were observed between the pixel and the guard ring in tests of single-pixel devices with a separation of 200 or 100 μm between the central electrode and the guard ring. The 122 and 136 keV photopeaks in 57 Co gamma-ray spectra were very well resolved with an energy resolution of up to 6.1% at 122 keV. Finally, a further reduction in gap size to 50 mu m is conceivable, but more process optimization is needed.
Perovskite CsPbBr 3 Single-Crystal Detector Operating at 10 10 Photons s -1 mm -2 for Ultra-High Flux X-ray Detection
Semiconductors for detecting hard radiation are confronted with considerable problems when operating at high photon fluxes. A perovskite CsPbBr 3 single-crystal detector capable of operating at X-ray fluxes of up to 10 10 photons s -1 mm -2 with beam area ≤ 0.25 mm 2 at 58.61 keV for current-mode X-ray detection is reported. The spectrometer-grade melt-grown CsPbBr 3 detectors show a gamma-ray energy resolution of ~7.5% at 122 keV for 57 Co and dark current as low as 4.3 nA (0.5 nA mm -2 ) at a reverse bias voltage of 200 V (118 V mm -1 ). The detector is tested at X-ray energies of 8.2, 10, and 58.61 keV at a synchrotron light source under a reverse bias voltage of up to 1000 V (588 V mm -1 ). Under a sufficiently high bias voltage and within several hundreds of seconds X-ray exposure, good photocurrent linearity (goodness of fit R 2 > 0.99) and reproducibility are obtained up to a flux of ≈10 10 photons s -1 mm -2 at beam area 0.25 mm 2 with Lower Limit of Detection of ≈10 5 photons s -1 mm -2 and Charge Collection Efficiency of ≈100% for 58.61 keV X-rays. Therefore, wide application of CsPbBr 3 detectors in high-flux X-ray detection is anticipated.
High power picosecond green and deep ultraviolet generations with an all-fiberized MOPA
We demonstrate high power picosecond green and deep ultraviolet (DUV) lasers based on an all-fiberized master oscillator power amplifier (MOPA). The main power amplifier is fabricated with a highly Yb-doped large mode area (LMA) silicate glass fiber. It delivers 75.2-W laser output at 1029 nm with a pulse repetition rate of 10 MHz and a pulse duration of 70 ps. With a lithium triborate (LBO) crystal, a 43.0-W green output at 514.5 nm has been achieved with a pulse duration of 55 ps. With a caesium lithium borate (CLBO) crystal, a 14.5-W picosecond DUV output at 257 nm has been generated, which is the highest power for the all-fiber based DUV laser, to the best of our knowledge.
Ultrafast optical control of surface and bulk magnetism in magnetic topological insulator/antiferromagnet heterostructure
Abstract Optical control of the magnetic properties in topological insulator systems is an important step in applying these materials in ultrafast optoelectronic and spintronic schemes. In this work, we report the experimental observation of photo-induced magnetization dynamics in the magnetically doped topological insulator (MTI)/antiferromagnet (AFM) heterostructure composed of Cr-(Bi,Sb) 2 Te 3 /CrSb. Through proximity coupling to the AFM layer, the MTI displays a dramatically enhanced magnetism, with robust perpendicular magnetic anisotropy. When subjected to intense laser irradiation, both surface and bulk magnetism of the MTI are weakened by laser-induced heating of the lattice, however, at the surface, the deleterious heat effect is compensated by the strengthening of Dirac-hole-mediated exchange coupling as demonstrated by an unconventional pump-fluence-dependent exchange-bias effect. Through theoretical analyses, the sizes of exchange coupling energies are estimated in the MTI/AFM bilayer structure. The fundamentally different mechanisms supporting the surface and bulk magnetic order in MTIs allow a novel and distinctive photo-induced transient magnetic state with antiparallel spin configuration, which broadens the understanding of the magnetization dynamics of MTIs under ultrashort and intense optical excitation.
Sensitivity and Detection Limit of Spectroscopic-Grade Perovskite CsPbBr 3 Crystal for Hard X-Ray Detection
We report spectroscopic-grade single crystal detectors can register the energies of individual X-ray interactions enabling photon-counting systems with superior resolution over traditional photoconductive X-ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy-discrimination X-ray photon-counting detection. Here, this work reports the deployment of a spectroscopic-grade CsPbBr 3 Schottky detector under reverse bias for continuum hard X-ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of approximate to ≈ 1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X-ray source is assessed in resolving the characteristic X-ray peak. The methodology in enhancing X-ray sensitivity by controlling the X-ray energies and flux, and voltage, is described. The X-ray sensitivity varies between a few tens to over 8000 μC Gy air -1 cm -2 . The detectable dose rate of the CsPbBr 3 detectors is as low as 0.02 nGy air s -1 in the energy discrimination configuration. Finally, the unbiased CsPbBr 3 device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr 3 single crystals to operate in "passive" self-powered X-ray detection mode and the X-ray sensitivity is estimated as 14 μC Gy air -1 cm -2 . The great potential of spectroscopic-grade CsPbBr 3 devices for X-ray photon-counting systems is anticipated in this work.