Search query: HIGH-PRESSURE STRUCTURAL TRANSFORMATIONS IN SEMICONDUCTOR NANOCRYSTALS

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abstract: Pressure induced structural transformations in semiconductor nanocrystals are examined. High pressure Raman spectroscopy, EXAFS, X ray diffraction, and optical absorption are discussed as methods for studying these transformations in CdSe, CdS, and Si nanocrystals. In these nanocrystal systems, each technique shows an elevation in solid solid structural transformation pressure as crystallite size decreases. By analogy with melting in nanocrystals, this elevation in transformation pressure is explained in terms of an increase in surface energy in the newly formed high pressure phase crystallites. The increase in surface energy is in turn the result of transition path induced changes in the shape of the nanocrystals. These changes convert spherical nanocrystals with low index, low energy surfaces into oblate or prolate crystallites with higher index, higher energy surfaces. The elevation in structural transformation pressure in nanocrystals is thus a kinetic rather than a thermodynamic phenomenon.
author_names: Sarah H Tolbert

A Paul Alivisatos
title: High pressure structural transformations in semiconductor nanocrystals.
venue: Annual review of physical chemistry
year: 1995

abstract: Abstract Pressure induced structural phase transitions have been studied in CdSe, CdS, InP and Si semiconductor nanocrystals. Nanocrystals transform via single nucleation of the phase transition with a kinetic barrier that increases in increasing cluster size. The structural transition path causes a shape change in the nanocrystals, which dictates the surface energy and thus the kinetic and thermodynamic stability of the transformed nanocrystal. These finite size effects can be used to tune the metastability of the nanocrystals versus pressure. Enhanced metastability allows structural and optical measurements in a regime inaccessible to the bulk solid, as well as possible recovery of the dense high pressure phase to atmospheric pressure.
author_names: Amy B Herhold

Chia-Chun Chen

C S Jun Johnson

Sarah H Tolbert

A Paul Alivisatos
title: Structural transformations and metastability in semiconductor nanocrystals
year: 1999

abstract: The melt spun (MS) Ti50Ni25Cu25 alloy and the Zr62Cu22Al10Fe5Dy1 bulk metallic glass (BMG) were subjected to high pressure torsion (HPT) X ray diffraction (XRD) measurements show a shift of the first diffraction halo to a low angle after HPT processing, which corresponds to an increase in the values of the radius of the first coordination sphere and the free volume. Direct density measurements confirmed an increase in free volume values. A special TEM procedure was used for a detailed study of the microstructure of both amorphous alloys after HPT processing. The study revealed the formation of a large density of shear bands (SBs) in both alloys. Nanocrystals are formed directly in shear bands as a result of strain induced nanocrystallization. Amorphous nanoclusters with a size of 20 nm are formed in an amorphous matrix surrounding the SBs in the HPT processed MS alloy Ti50Ni25Cu25. The formation of nanoclusters was not observed in BMG Zr62Cu22Al10Fe5Dy1 after HPT processing.
author_names: Dmitry V Gunderov

Evgeniy Boltynjuk

Evgeniy V Ubyivovk

Anna Churakova

Galina Abrosimova

V D Sitdikov

Askar R Kilmametov

Ruslan Z Valiev
title: High pressure torsion induced structural transformations in Ti and Zr based amorphous alloys
year: 2018

abstract: X ray diffraction was used to monitor the structure of 45 A diameter CdSe nanocrystals as they transformed repeatedly between fourfold and sixfold coordinated crystal structures. Simulations of the diffraction patterns reveal that a shape change occurs as the crystals transform. They also show that stacking faults are generated in the transition from the high to the low pressure phase. The shape change and stacking fault generation place significant constraints on the possible microscopic mechanism of the phase transition.
title: Shape change as an indicator of mechanism in the high pressure structural transformations of CdSe nanocrystals
venue: Physical review letters
year: 2000

abstract: Understanding structural stability and phase transformation of nanoparticles under high pressure is of great scientific interest, as it is one of the crucial factors for design, synthesis, and application of materials. Even though high pressure research on nanomaterials has been widely conducted, their shape dependent phase transition behavior still remains unclear. Examples of phase transitions of CdS nanoparticles are very limited, despite the fact that it is one of the most studied wide band gap semiconductors. In this study, we have employed in situ synchrotron wide angle X ray scattering (WAXS) and transmission electron microscopy (TEM) to investigate the high pressure behaviors of CdS nanoparticles as a function of particle shapes. We observed that CdS nanoparticles transform from wurtzite to rocksalt phase at elevated pressure in comparison to their bulk counterpart. Phase transitions also vary with particle shape rod shaped particles show a partially reversible phase transition and the onset of the structural phase transition pressure decreases with decreasing surface to volume ratios, while spherical particles undergo irreversible phase transition with relatively low phase transition pressure. In addition, TEM images of spherical particles exhibited sintering induced morphology change after high pressure compression. Calculations of the bulk modulus reveals that spheres are more compressible than rods in the wurtzite phase. These results indicate that the shape of the particle plays an important role in determining their high pressure properties. Our study provides important insights into the understanding of the phase structure property relationship, which may guide future design and synthesis of nanoparticles for promising applications.
author_names: Lingyao Meng

J Matthew D Lane

Luke Baca

Jackie Tafoya

Tommy Ao

Brian Stoltzfus

Marcus D Knudson

Dane Morgan

Kevin N Austin

Changyong Park

Paul Chow

Yuming Xiao

Ruipeng Li

Yang Qin

Hongyou Fan
title: Shape Dependence of Pressure Induced Phase Transition in CdS Semiconductor Nanocrystals.
venue: Journal of the American Chemical Society
year: 2020

abstract: We have observed and identified two crystalline peaks at ~270 cm 1 and ~400 cm 1 in nanocrystalline silicon during both compression and decompression. We attribute the first peak to the orthorhombic Imma phase (Si XI) and the second one to the TO mode of the silicon metallic b Sn phase (Si II) Also, in the process of decompression, we observed a first order like transition from a highly coordinated metallic crystalline b Sn phase to the normal tetrahedral LDA phase of the amorphous semiconductor. When the pressure is completely relieved, the powder regains its initial crystalline structure, which suggests that the amorphous phase has retained some structural features of the parent crystalline phase.We have observed and identified two crystalline peaks at ~270 cm 1 and ~400 cm 1 in nanocrystalline silicon during both compression and decompression. We attribute the first peak to the orthorhombic Imma phase (Si XI) and the second one to the TO mode of the silicon metallic b Sn phase (Si II) Also, in the process of decompression, we observed a first order like transition from a highly coordinated metallic crystalline b Sn phase to the normal tetrahedral LDA phase of the amorphous semiconductor. When the pressure is completely relieved, the powder regains its initial crystalline structure, which suggests that the amorphous phase has retained some structural features of the parent crystalline phase.
author_names: Nikolay N Ovsyuk

Sergey G Lyapin
title: Raman spectra of Si nanocrystals under high pressure: Metallization and solid state amorphization
year: 2020

abstract: Here, we report comprehensive studies on the high pressure structural and electrical transport properties of the layered transition metal chalcogenide (Cr2S3) up to 36.3 GPa. A structural phase transition was observed in the rhombohedral Cr2S3 near 16.5 GPa by the synchrotron angle dispersive X ray diffraction measurement using a diamond anvil cell. Through in situ resistance measurement, the electric resistance value was detected to decrease by an order of three over the pressure range of 7 15 GPa coincided with the structural phase transition. Measurements on the temperature dependence of resistivity indicate that it is a semiconductor to metal transition in nature. The results were also confirmed by the electronic energy band calculations. Above results may shed a light on optimizing the performance of Cr2S3 based applications under extreme conditions.
author_names: Chunyu Li

Feng Ke

Qingyang Hu

Zhenhai Yu

Jinggeng Zhao

Zhi-Qian Chen

Hao Yan
title: Correlated structural and electronic phase transformations in transition metal chalcogenide under high pressure
year: 2016

abstract: Considering the large lattice mismatch induced interface strain between nonepitaxial grown monocrystalline semiconductor shell and metal core, we studied the solid state phase transformation of such nonepitaxial grown Au@CdS core/shell NCs under high pressure in this paper. Consistent with HRTEM characterizations, the high resolution Raman spectra and synchrotron angle dispersive X ray diffraction (ADXRD) spectra evolution were utilized to investigate the hydrostatic pressure (0 24 GPa) induced gradual phase transformation. Due to the strong lattice interactions between Au core and CdS shell, the different behavior and improved stability under high pressure appeared compared to single quantum dots (QDs)
author_names: Muwei Ji

Hongzhi Wang

Yu Gong

Hai-Xia Cheng

Lirong Zheng

Xinyuan Li

Liu Ying Huang

Jia Liu

Zhihua Nie

Qiaoshi Zeng

Meng Xue Xu

Jiajia Liu

Xiaoxu Wang

Ping Qian

Caizhen Zhu

Jin Wang

Xiao-Da Li

Jiatao Zhang
title: High Pressure Induced in Situ Solid State Phase Transformation of Nonepitaxial Grown Metal@Semiconductor Nanocrystals.
venue: The journal of physical chemistry letters
year: 2018

author_names: Juanita Wickham

Amy B Herhold

A Paul Alivisatos
title: Erratum: Shape Change as an Indicator of Mechanism in the High Pressure Structural Transformations of CdSe Nanocrystals [Phys. Rev. Lett. 84, 923 (2000)
year: 2000

abstract: Abstract TiO2 and ZnO, two semiconductors with promising optical properties, are considered as potential candidates for solar and photocatalytic applications. Although chemical methods have been primarily used to enhance the optical properties of these oxides, the current authors recently reported enhanced photocatalytic performance of pure TiO2 and ZnO by plastic straining due to the generation of high pressure phases and oxygen vacancies. In this study, to improve the optical properties further, large fractions of ZnO/TiO2 interphase boundaries are also introduced by application of high pressure torsion (HPT) straining to a mixture of anatase TiO2 and wurtzite ZnO powders. It was found that the amounts of oxygen vacancies and nanograined high pressure TiO2 II and rocksalt ZnO phases increase with increasing plastic strain. Moreover, due to the plastic strain effect, the rutile TiO2 phase is formed at room temperature, which is at least 600 K below the reported anatase to rutile transition temperature. These structural features, together with the formation of large fraction of interphase boundaries, lead to electron spin resonance, optical bandgap narrowing, diminishing of the band to band photoluminescence and thus, improvement of photocatalytic hydrogen generation. Despite improvements in the photocatalytic activity of TiO2 ZnO composites after large straining, photocatalytic activity becomes poor by processing at ultra large strains due to the significant reduction in crystallinity.
author_names: Jacqueline Hidalgo-Jimenez

Qing Bo Wang

Kaveh Edalati

Jorge M Cubero-Sesin

Hadi Razavi-Khosroshahi

Yoshifumi Ikoma

Dionisio Gutierrez-Fallas

Fernando A Dittel-Meza

Juan Carlos Rodriguez-Rufino

Masayoshi Fuji

Zenji Horita
title: Phase transformations, vacancy formation and variations of optical and photocatalytic properties in TiO2 ZnO composites by high pressure torsion
year: 2020