Publications

Phase-change materials are important for optical and electronic computing memory. Ge–Sb–Te (GST) is one of the important phase-change materials and has been studied extensively for fast, reversible, and non-volatile electronic phase-change memory. GST exhibits structural transformations from amorphous to metastable fcc at ~150 ℃ and fcc to hcp at ~300 ℃. The investigation of the structural, microstructural, and microchemical changes with high-temporal resolution during heating is crucial to gain insights on the changes that materials undergo during phase transformations. The as-deposited GST film has amorphous island morphology which transform to the metastable fcc phase at ~130 ℃. The second-phase transformation, from fcc to hexagonal, is observed at ~170 ℃. While the as-deposited amorphous islands show a homogeneous distribution of Ge, Sb and Te, these islands boundaries become Ge-rich after heating. Morphological and structural evolutions were captured during heating inside an aberration corrected environmental TEM equipped with a high-speed camera under a low-dose conditions to minimize beam-induced changes in the samples. Microchemical studies were carried out employing ChemiSTEM technique in probe-corrected mode with a monochromated beam.

Ge2Sb2Te5 (GST-225) has been the most used active material in nonvolatile phase-change memory devices. Understanding the kinetics and dynamics involved in crystallization is critical for the optimization of materials and devices. A GST-225 thin film of 20 nm thickness was prepared by sputtering directly onto a Protochip and left uncapped and exposed to atmosphere for approximately 1 year. Early stages of crystallization and growth of the film have been studied inside the TEM from room temperature to 140 °C. The morphological and structural transformations have been studied by a Cs-corrected environmental TEM, and images have been recorded using a high-speed low electron dose camera (Gatan K3 IS). The amorphous to crystalline transformation has been observed at ~35 °C. From the large field, high-resolution images obtained using the Gatan K3 IS camera early crystallization can be detected and nucleation rates and growth velocities can be obtained.

Germanium antimony telluride has been the most used and studied phase-change material for electronic memory due to its suitable crystallization temperature, amorphous to crystalline resistance contrast, and stability of the amorphous phase. In this paper, the segregation of Ge in a Ge₂Sb₂Te₅ film of 30 nm thickness during heating inside the transmission electron microscope was observed and characterized. Furthermore, Ge₂Sb₂Te₅ film was deposited using sputtering on a Protochips Fusion holder and left uncapped in atmosphere for about four months. Oxygen incorporated within the film played a significant role in the chemical segregation observed which resulted in amorphous Ge-O island boundaries and Sb and Te rich crystalline domains. Such composition changes can occur when the phase-change material interfaces insulating oxide layers in an integrated device and can significantly impact its electrical and thermal properties.

Cryogenic transmission electron microscopy is simply transmission electron microscopy conducted on specimens that are cooled in the microscope. The target temperature of the specimen might range from just below ambient temperature to less than 4 K. In general, as the temperature decreases, cost increases, especially below -77°C when liquid He is required. We have two reasons for wanting to cool the specimen - improving stability of the material or observing a material whose properties change at lower temperatures. Both types of study have a long history. The cause of excitement in this field today is that we have a perfect storm of research activity - electron microscopes are almost stable with minimal drift (we can correct what drift there is), we can prepare specimens from the bulk or build them up, we have spherical-aberration-corrected lenses and monochromated beams, we have direct-electron-detector cameras, and computers are becoming powerful enough to handle all the data we produce.

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Lead halide perovskites are increasingly considered for applications beyond photovoltaics, for example, light emission and detection, where an ability to pattern and prototype microscale geometries can facilitate the incorporation of this class of materials into devices. Here we demonstrate laser direct write of lead halide perovskites, a remarkably simple procedure that takes advantage of the inverse dependence between perovskite solubility and temperature by using a laser to induce localized heating of an absorbing substrate. We demonstrate arbitrary pattern formation of crystalline CH3NH3PbBr3 on a range of substrates and fabricate and characterize a microscale photodetector using this approach. This direct write methodology provides a path forward for the prototyping and production of perovskite-based devices.

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We employed an in situ electrochemical cell in the transmission electron microscope (TEM) together with ex situ time-of-flight, secondary-ion mass spectrometry (TOF-SIMS) depth profiling, and FIB-helium ion scanning microscope (HIM) imaging to detail the structural and compositional changes associated with Na/Na+ charging/discharging of 50 and 100 nm thin films of Sb. TOF-SIMS on a partially sodiated 100 nm Sb film gives a Na signal that progressively decreases toward the current collector, indicating that sodiation does not proceed uniformly. This heterogeneity will lead to local volumetric expansion gradients that would in turn serve as a major source of intrinsic stress in the microstructure. In situ TEM shows time-dependent buckling and localized separation of the sodiated films from their TiN-Ge nanowire support, which is a mechanism of stress-relaxation. Localized horizontal fracture does not occur directly at the interface, but rather at a short distance away within the bulk of the Sb. HIM images of FIB cross sections taken from sodiated half-cells, electrically disconnected, and aged at room temperature, demonstrate nonuniform film swelling and the onset of analogous through-bulk separation. TOF-SIMS highlights time-dependent segregation of Na within the structure, both to the film-current collector interface and to the film surface where a solid electrolyte interphase (SEI) exists, agreeing with the electrochemical impedance results that show time-dependent increase of the films' charge transfer resistance. We propose that Na segregation serves as a secondary source of stress relief, which occurs over somewhat longer time scales.

Materials such as tin (Sn) and silicon that alloy with lithium (Li) have attracted renewed interest as anode materials in Li-ion batteries. Although their superior capacity to graphite and other intercalation materials has been known for decades, their mechanical instability due to extreme volume changes during cycling has traditionally limited their commercial viability. This limitation is changing as processes emerge that produce nanostructured electrodes. The nanostructures can accommodate the repeated expansion and contraction as Li is inserted and removed without failing mechanically. Recently, one such nano-manufacturing process, which is capable of depositing coatings of Sn “nanoneedles” at low temperature with no template and at industrial scales, has been described. The present work is concerned with observations of the lithiation and delithiation behavior of these Sn nanoneedles during in situ experiments in the transmission electron microscope, along with a brief review of how in situ TEM experiments have been used to study the lithiation of Li-alloying materials. Individual needles are successfully lithiated and delithiated in solid-state half-cells against a Li-metal counter-electrode. The microstructural evolution of the needles is discussed, including the transformation of one needle from single-crystal Sn to polycrystalline Sn–Li and back to single-crystal Sn.

Thin tantalum films have been studied during in situ heating in a transmission electron microscope. Diffraction patterns from the as-deposited films were typical of amorphous materials. Crystalline grains were observed to form when the specimen was annealed in situ at 450 °C. Particular attention was addressed to the formation and growth of grains with the face-centered cubic (fcc) crystal structure. These observations are discussed in relation to prior work on the formation of fcc Ta by deformation and during thin film deposition.

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One-dimensional (1D) nanostructures, often referred to as nanowires, have attracted considerable attention due to their unique mechanical, chemical, and electrical properties. Although numerous novel technological applications are being proposed for these structures, many of the processes used to synthesize these materials involve a vapor phase and require high temperatures and long growth times. Potentially faster methods requiring templates, such as anodized aluminum oxide, involve multiple fabrication steps, which would add significantly to the cost of the final material and may preclude their widespread use. In the present study, it is shown that template-free electrodeposition from an alkaline solution can produce arrays of Sn nanoneedles directly onto Cu foil substrates. This electrodeposition process occurs at 55 C; it is proposed that the nanoneedles grow via a catalyst-mediated mechanism. In such a process, the growth is controlled at the substrate/nanostructure interface rather than resulting from random plating-induced defects such as dendrites or aging defects such as tin whiskers. There are multiple potential applications for 1D Sn nanostructures - these include anodes in lithium-ion and magnesium-ion batteries and as thermal interface materials. To test this potential, type 2032 lithium-ion battery button cells were fabricated using the electrodeposited Sn. These cells showed initial capacities as high as 850 mAh/g and cycling stability for over 200 cycles. © 2013 Springer Science+Business Media New York.