We present a detailed investigation of novel strain-driven semiconductor nanostructures. Our examinations include self-assembled nanoholes, lateral quantum-dot (QD) molecules, and rolled-up nanotubes. We overgrow InAs QDs with GaAs and apply atomically precise in situ etching to fabricate homogeneous arrays of nanometer-sized holes with diameters of 40 to 60 nm and depths up to 6.2 nm. The structural properties of the nanoholes can be precisely tuned by changing the QD capping thickness and the in situ etching time. We show that strain fields surrounding the buried quantum dots drive the nanohole formation process. We overgrow the nanoholes with 0.2- to 2.5-ML InAs and observe the formation of compact lateral InAs QD molecules. The number of QDs involved in a lateral QD molecule can be tuned from two to six by changing the growth temperature. Our systematic photoluminescence study documents the QD molecule formation process step by step and helps to interpret our structural results. We also present the fabrication of laterally aligned lateral QD bimolecules by growing InGaAs on a GaAs [001] substrate patterned with a square array of nanometer sized holes. Charge carriers in such bimolecules might serve as quantum gates in a future semiconductor based quantum computer. Furthermore, we release strained semiconductor bilayers from their surface to fabricate individual rolled-up semiconductor micro- and nanotubes. We control the diameter of strain-driven In(Ga)As-GaAs tubes from the nanometer to micrometer range by simply changing the layer thicknesses and built-in strain. We propose to roll in metal strip lines to fabricate nanocoils and nanotransformers. To support our proposition, we fabricate homogeneous single and twin GaInP tubes. We present a straight GaInP microtube of more than 2 mm in length and a length-to-diameter ratio of about 2000, thus, elucidating the great potential of this technology.

Self-assembled nanoholes, lateral quantum-dot molecules, and rolled-up nanotubes

Detection of trace tetracycline in fish via synchronous fluorescence quenching with carbon quantum dots coated with molecularly imprinted silica.

This article introduces a new scintillator material Rb2AgBr3 that has good luminescence properties from the perspective of synthesis, characterization, and theoretical calculation. A scintillation detector is constructed, and successful X‐ray static and dynamic imaging is achieved, pointing to a promising direction for X‐ray indirect detectors in future.

Metal Halide Scintillators with Fast and Self-Absorption-Free Defect-Bound Excitonic Radioluminescence for Dynamic X-Ray Imaging

In the present work, cesium iodide (CsI) thin films of different thickness have been prepared by thermal evaporation technique. The crystallite size and grain size of these films are compared by using X-ray diffraction (XRD) profile analysis as well as by transmission electron microscopy (TEM) counting, respectively. These two methods provide less deviation between crystallite size and grain size in the case of thin CsI films of 4 nm, but there is comparatively large difference in case of thicker CsI films (20 nm, 100 nm and 500 nm). It indicates that dislocations are arranged in a configuration which causes small orientational difference between two adjacent coherent regions. The size obtained from XRD corresponds to two separate regions, whereas in the TEM micrograph the two regions may seem to correspond one region particularly in case of thicker films. Other physical parameters such as strain, stress and deformation energy density are also estimated precisely for the prominent XRD peaks of thicker CsI films in the range $2\theta = 20^{0}-80^{0}$ by using a modified Williamson-Hall (W-H) analysis assuming uniform deformation model (UDM), uniform deformation stress model (UDSM) and uniform deformation energy density model (UDEDM).

Structural characterization of as-deposited cesium iodide films studied by X-ray diffraction and transmission electron microscopy techniques

Scintillators, which can convert high-energy ionizing radiation into visible light, have been serving as the core component in radiation detectors for more than a century of history. To address the increasing application demands along with the concern on nuclear security, various strategies have been proposed to develop a next-generation scintillator with a high performance in past decades, among which the novel approach via structure control has received great interest recently due to its high feasibility and efficiency. Herein, the concept of "structure engineering" is proposed for the exploration of this type of scintillators. Via internal or external structure design with size ranging from micro size to macro size, this promising strategy cannot only improve scintillator performance, typically radiation stopping power and light yield, but also extend its functionality for specific applications such as radiation imaging and therapy, opening up a new range of material candidates. The research and development of various types of structured scintillators are reviewed. The current state-of-the-art progresses on structure design, fabrication techniques, and the corresponding applications are discussed. Furthermore, an outlook focusing on the current challenges and future development is proposed.

Structured Scintillators for Efficient Radiation Detection

An improved microchip thin film transformer formed by vacuum evaporation and sputtering

Although there are many new scintillators being developed recently, CsI: Tl is still very efficient among them. The fluorescent lifetime is a very important parameter of CsI: Tl thin film and two series of experiments have been conducted to learn about it. Our experiments, however, have demonstrated that the deposition rate and the codoping of Eu 2+ will significantly influence its fluorescent lifetime. In order to increase the efficiency of the imaging system, we intend to obtain a higher fluorescent lifetime for CsI: Tl thin film by controlling these two conditions.

Study on the effect of deposition rate and concentration of Eu on the fluorescent lifetime of CsI: Tl thin film

CsI(Tl) scintillator films with columnar structure are widely applied as the conversion screens for the indirect X-ray imaging. In this work, CsI(Tl) films with different micron thickness were fabricated on glass substrates by the thermal deposition method under the same deposition conditions. The influence of film thickness on the microstructure and crystalline property of the films was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The photoluminescent spectra of the films were measured, which appeared by the bimodal distribution peaking at 550 nm and 740 nm respectively. The radioluminescence and imaging performances were observed by the micron thick films coupled with CCD camera system under X-ray exposure conditions.

Fabrication and Performance of Micron Thick CsI(Tl) Films for X-Ray Imaging Application

Structure and scintillation properties of CsI(Tl) films on Si single crystal substrates

A thermo-optical coefficient (TOC) test platform based on FILMeasure-20 was designed and the thermal coefficient of hydrogenated amorphous silicon (a-Si:H) thin films material at 1330 nm was tested. a-Si:H were deposited on the quartz glass using a plasma-enhanced chemical vapor deposition (PECVD) system. Fourier transform infrared spectrometer (FTIR) was used to characterize the infrared spectral feature of films. The hydrogen content of films was influenced by different radio frequency (RF) power and deposition pressure conditions according to the FTIR spectra and theoretical analysis, and the thermo-optic effect of a-Si:H varied with temperature characteristics. Experimental results indicated that selecting the appropriate process conditions to prepare a-Si:H films can effectively increase or avoid the impact of thermo-optical effect on the optical devices.

Zhiyong Zhong

Papers

An improved microchip thin film transformer formed by vacuum evaporation and sputtering

Study on the effect of deposition rate and concentration of Eu on the fluorescent lifetime of CsI: Tl thin film

Fabrication and Performance of Micron Thick CsI(Tl) Films for X-Ray Imaging Application

Structure and scintillation properties of CsI(Tl) films on Si single crystal substrates

Study on the effect of process conditions on the thermo-optic coefficient of amorphous silicon films