Double-shelling AgInS2nanocrystals with GaSx/ZnS to make them emit bright and stable excitonic luminescence.

This paper presents the successful synthesis of AgInS2nanocrystals (NCs) double-shelled with GaSxand ZnS for emitting bright and narrow excitonic luminescence from AgInS2core NCs. Additionally, the AgInS2/GaSx/ZnS NCs with a core/double-shell structure have demonstrated high chemical and photochemical stability. The AgInS2/GaSx/ZnS NCs were prepared via three steps: (i) synthesis of AgInS2core NCs by solvothermal method at 200 °C for 30 min, (ii) shelling GaSxon AgInS2core NCs at 280 °C for 60 min to produce the AgInS2/GaSxcore/shell structure, and (iii) the outermost ZnS shelling at 140 °C for 10 min. The synthesized NCs were characterized in detail by using appropriate techniques such as x-ray diffraction, transmission electron microscopy, and optical spectroscopies. The luminescence evolution of the synthesized NCs is as follows: from the broad spectrum (peaking at 756 nm) of the AgInS2core NCs to become the narrow excitonic emission (at 575 nm) prominent beside the broad one after shelling with GaSx, then only the bright excitonic luminescence (at 575 nm) without broad emission after double-shelling with GaSx/ZnS. The double-shell has made the AgInS2/GaSx/ZnS NCs not only remarkably enhance their luminescence quantum yield (QY) up to ∼60% but also maintain the narrow excitonic emission stably for a long-term storage over 12 months. The outermost ZnS shell is believed to play a key role in enhancing QY and protecting AgInS2and AgInS2/GaSxfrom certain damage.

Thermally propagated Al contacts on SiGe nanowires characterized by electron beam induced current in a scanning transmission electron microscope.

Here, we use electron beam induced current (EBIC) in a scanning transmission electron microscope to characterize the structure and electronic properties of Al/SiGe and Al/Si-rich/SiGe axial nanowire heterostructures fabricated by thermal propagation of Al in a SiGe nanowire. The two heterostructures behave as Schottky contacts with different barrier heights. From the sign of the beam induced current collected at the contacts, the intrinsic semiconductor doping is determined to be n-type. Furthermore, we find that the silicon-rich double interface presents a lower barrier height than the atomically sharp SiGe/Al interface. With an applied bias, the Si-rich region delays the propagation of the depletion region and presents a reduced free carrier diffusion length with respect to the SiGe nanowire. This behaviour could be explained by a higher residual doping in the Si-rich area. These results demonstrate that scanning transmission electron microscopy EBIC is a powerful method for mapping and quantifying electric fields in micrometer- and nanometer-scale devices.

Nanometer-Scale Ge-Based Adaptable Transistors Providing Programmable Negative Differential Resistance Enabling Multivalued Logic.

The functional diversification and adaptability of the elementary switching units of computational circuits are disruptive approaches for advancing electronics beyond the static capabilities of conventional complementary metal-oxide-semiconductor-based architectures. Thereto, in this work the one-dimensional nature of monocrystalline and monolithic Al-Ge-based nanowire heterostructures is exploited to deliver charge carrier polarity control and furthermore to enable distinct programmable negative differential resistance at runtime. The fusion of electron and hole conduction together with negative differential resistance in a universal adaptive transistor may enable energy-efficient reconfigurable circuits with multivalued operability that are inherent components of emerging artificial intelligence electronics.

Effect of Nitrogen Doping on the Crystallization Kinetics of Ge2Sb2Te5.

Among the phase change materials, Ge2Sb2Te5 (GST-225) is the most studied and is already integrated into many devices. N doping is known to significantly improve some key characteristics such as the thermal stability of materials and the resistance drift of devices. However, the origin, at the atomic scale, of these alterations is rather elusive. The most important issue is to understand how N doping affects the crystallization characteristics, mechanisms and kinetics, of GST-225. Here, we report the results of a combination of in situ and ex situ transmission electron microscopy (TEM) investigations carried out on specifically designed samples to evidence the influence of N concentration on the crystallization kinetics and resulting morphology of the alloy. Beyond the known shift of the crystallization temperature and the observation of smaller grains, we show that N renders the crystallization process more "nucleation dominated" and ascribe this characteristic to the increased viscosity of the amorphous state. This increased viscosity is linked to the mechanical rigidity and the reduced diffusivity resulting from the formation of Ge-N bonds in the amorphous phase. During thermal annealing, N hampers the coalescence of the crystalline grains and the cubic to hexagonal transition. Making use of AbStrain, a recently invented TEM-based technique, we evidence that the nanocrystals formed from the crystallization of N-doped amorphous GST-225 are under tension, which suggests that N is inserted in the lattice and explains why it is not found at grain boundaries. Globally, all these results demonstrate that the origin of the effect of N on the crystallization of GST-225 is not attributed to the formation of a secondary phase such as a nitride, but to the ability of N to bind to Ge in the amorphous and crystalline phases and to unbind and rebind with Ge along the diffusion path of this atomic species during annealing.

Reversible Al Propagation in Si x Ge1-x Nanowires: Implications for Electrical Contact Formation.

While reversibility is a fundamental concept in thermodynamics, most reactions are not readily reversible, especially in solid-state physics. For example, thermal diffusion is a widely known concept, used among others to inject dopants into the substitutional positions in the matrix and improve device properties. Typically, such a diffusion process will create a concentration gradient extending over increasingly large regions, without possibility to reverse this effect. On the other hand, while the bottom-up growth of semiconducting nanowires is interesting, it can still be difficult to fabricate axial heterostructures with high control. In this paper, we report a thermally assisted partially reversible thermal diffusion process occurring in the solid-state reaction between an Al metal pad and a Si x Ge1-x alloy nanowire observed by in situ transmission electron microscopy. The thermally assisted reaction results in the creation of a Si-rich region sandwiched between the reacted Al and unreacted Si x Ge1-x part, forming an axial Al/Si/Si x Ge1-x heterostructure. Upon heating or (slow) cooling, the Al metal can repeatably move in and out of the Si x Ge1-x alloy nanowire while maintaining the rodlike geometry and crystallinity, allowing to fabricate and contact nanowire heterostructures in a reversible way in a single process step, compatible with current Si-based technology. This interesting system is promising for various applications, such as phase change memories in an all crystalline system with integrated contacts as well as Si/Si x Ge1-x /Si heterostructures for near-infrared sensing applications.

Plasmon-Driven Hot Electron Transfer at Atomically Sharp Metal-Semiconductor Nanojunctions.

Recent advances in guiding and localizing light at the nanoscale exposed the enormous potential of ultrascaled plasmonic devices. In this context, the decay of surface plasmons to hot carriers triggers a variety of applications in boosting the efficiency of energy-harvesting, photocatalysis, and photodetection. However, a detailed understanding of plasmonic hot carrier generation and, particularly, the transfer at metal-semiconductor interfaces is still elusive. In this paper, we introduce a monolithic metal-semiconductor (Al-Ge) heterostructure device, providing a platform to examine surface plasmon decay and hot electron transfer at an atomically sharp Schottky nanojunction. The gated metal-semiconductor heterojunction device features electrostatic control of the Schottky barrier height at the Al-Ge interface, enabling hot electron filtering. The ability of momentum matching and to control the energy distribution of plasmon-driven hot electron injection is demonstrated by controlling the interband electron transfer in Ge, leading to negative differential resistance.

Stimulated Raman Scattering in Ge Nanowires.

Investigating group-IV-based photonic components is a very active area of research with extensive interest in developing complementary metal-oxide-semiconductor (CMOS) compatible light sources. However, due to the indirect band gap of these materials, effective light-emitting diodes and lasers based on pure Ge or Si cannot be realized. In this context, there is considerable interest in developing group-IV based Raman lasers. Nevertheless, the low quantum yield of stimulated Raman scattering in Si and Ge requires large device footprints and high lasing thresholds. Consequently, the fabrication of integrated, energy-efficient Raman lasers is challenging. Here, we report the systematic investigation of stimulated Raman scattering (SRS) in Ge nanowires (NWs) and axial Al-Ge-Al NW heterostructures with Ge segments that come into contact with self-aligned Al leads with abrupt metal-semiconductor interfaces. Depending on their geometry, these quasi-one-dimensional (1D) heterostructures can reassemble into Ge nanowires, Ge nanodots, or Ge nanodiscs, which are monolithically integrated within monocrystalline Al (c-Al) mirrors that promote both optical confinement and effective heat dissipation. Optical mode resonances in these nanocavities support in SRS thresholds as low as 60 kW/cm2. Most notably, our findings provide a platform for elucidating the high potential of future monolithically integrated, nanoscale low-power group-IV-based Raman lasers.

Highly Transparent Contacts to the 1D Hole Gas in Ultrascaled Ge/Si Core/Shell Nanowires.

Semiconductor-superconductor hybrid systems have outstanding potential for emerging high-performance nanoelectronics and quantum devices. However, critical to their successful application is the fabrication of high-quality and reproducible semiconductor-superconductor interfaces. Here, we realize and measure axial Al-Ge-Al nanowire heterostructures with atomically precise interfaces, enwrapped by an ultrathin epitaxial Si layer further denoted as Al-Ge/Si-Al nanowire heterostructures. The heterostructures were synthesized by a thermally induced exchange reaction of single-crystalline Ge/Si core/shell nanowires and lithographically defined Al contact pads. Applying this heterostructure formation scheme enables self-aligned quasi one-dimensional crystalline Al leads contacting ultrascaled Ge/Si segments with contact transparencies greater than 96%. Integration into back-gated field-effect devices and continuous scaling beyond lithographic limitations allows us to exploit the full potential of the highly transparent contacts to the 1D hole gas at the Ge-Si interface. This leads to the observation of ballistic transport as well as quantum confinement effects up to temperatures of 150 K. Low-temperature measurements reveal proximity-induced superconductivity in the Ge/Si core/shell nanowires. The realization of a Josephson field-effect transistor allows us to study the subgap structure caused by multiple Andreev reflections. Most importantly, the absence of a quantum dot regime indicates a hard superconducting gap originating from the highly transparent contacts to the 1D hole gas, which is potentially interesting for the study of Majorana zero modes. Moreover, underlining the importance of the proposed thermally induced Al-Ge/Si-Al heterostructure formation technique, our system could contribute to the development of key components of quantum computing such as gatemon or transmon qubits.

In Situ Transmission Electron Microscopy Analysis of Aluminum-Germanium Nanowire Solid-State Reaction.

To fully exploit the potential of semiconducting nanowires for devices, high quality electrical contacts are of paramount importance. This work presents a detailed in situ transmission electron microscopy (TEM) study of a very promising type of NW contact where aluminum metal enters the germanium semiconducting nanowire to form an extremely abrupt and clean axial metal-semiconductor interface. We study this solid-state reaction between the aluminum contact and germanium nanowire in situ in the TEM using two different local heating methods. Following the reaction interface of the intrusion of Al in the Ge nanowire shows that at temperatures between 250 and 330 °C the position of the interface as a function of time is well fitted by a square root function, indicating that the reaction rate is limited by a diffusion process. Combining both chemical analysis and electron diffraction we find that the Ge of the nanowire core is completely exchanged by the entering Al atoms that form a monocrystalline nanowire with the usual face-centered cubic structure of Al, where the nanowire dimensions are inherited from the initial Ge nanowire. Model-based chemical mapping by energy dispersive X-ray spectroscopy (EDX) characterization reveals the three-dimensional chemical cross-section of the transformed nanowire with an Al core, surrounded by a thin pure Ge (∼2 nm), Al2O3 (∼3 nm), and Ge containing Al2O3 (∼1 nm) layer, respectively. The presence of Ge containing shells around the Al core indicates that Ge diffuses back into the metal reservoir by surface diffusion, which was confirmed by the detection of Ge atoms in the Al metal line by EDX analysis. Fitting a diffusion equation to the kinetic data allows the extraction of the diffusion coefficient at two different temperatures, which shows a good agreement with diffusion coefficients from literature for self-diffusion of Al.

Demonstration of a 2 × 2 programmable phase plate for electrons.

First results on the experimental realisation of a 2 × 2 programmable phase plate for electrons are presented. The design consists of an array of electrostatic elements that influence the phase of electron waves passing through 4 separately controllable aperture holes. This functionality is demonstrated in a conventional transmission electron microscope operating at 300 kV and results are in very close agreement with theoretical predictions. The dynamic creation of a set of electron probes with different phase symmetry is demonstrated, thereby bringing adaptive optics in TEM one step closer to reality. The limitations of the current design and how to overcome these in the future are discussed. Simulations show how further evolved versions of the current proof of concept might open new and exciting application prospects for beam shaping and aberration correction.

Effect of the nanowire diameter on the linearity of the response of GaN-based heterostructured nanowire photodetectors.

Nanowire photodetectors are investigated because of their compatibility with flexible electronics, or for the implementation of on-chip optical interconnects. Such devices are characterized by ultrahigh photocurrent gain, but their photoresponse scales sublinearly with the optical power. Here, we present a study of single-nanowire photodetectors displaying a linear response to ultraviolet illumination. Their structure consists of a GaN nanowire incorporating an AlN/GaN/AlN heterostructure, which generates an internal electric field. The activity of the heterostructure is confirmed by the rectifying behavior of the current-voltage characteristics in the dark, as well as by the asymmetry of the photoresponse in magnitude and linearity. Under reverse bias (negative bias on the GaN cap segment), the detectors behave linearly with the impinging optical power when the nanowire diameter is below a certain threshold (≈80 nm), which corresponds to the total depletion of the nanowire stem due to the Fermi level pinning at the sidewalls. In the case of nanowires that are only partially depleted, their nonlinearity is explained by a nonlinear variation of the diameter of their central conducting channel under illumination.

Mobile phone use among female entertainment workers in Cambodia: an observation study.

BACKGROUND: Text or voice messages containing health behavior change content may be an inexpensive, discreet, sustainable and scalable way to reach populations at high risk for HIV. In Cambodia, one of the important high-risk populations is female entertainment workers (FEWs). This ethnographic study aims to explore typical phone use, examining patterns and behaviors that may influence the design of future mHealth interventions. METHODS: The study consisted of one 8-hour non-participant observation session for 15 randomly sampled FEWs. Observations focused on capturing normal daily use of mobile devices. Observation checklists were populated by observers during the observations and a post-observation survey was conducted. Findings were discussed with Cambodian HIV outreach workers and HIV research fellows and their interpretations are summarized below. RESULTS: In this ethnographic study, all 15 participants made calls, checked the time and received research-related texts. More than half (n=8) of the participants engaged in texting to a non-research recipient. About half (n=7) went on Facebook (FB) and some (n=5) listened to music and looked at their FB newsfeed. Fewer played a mobile game, posted a photo to FB, went on YouTube, used FB chat/messenger, watched a video on FB, played a game on FB, used FB call/voice chat, looked at their phone's background or used the LINE app. Fewer still shared their phones, left them unattended, added airtime or changed their SIM cards. When participants received a research text message, most did not share the text message with anyone, did not ask for help deciphering the message and did not receive help composing a response. Notable themes from observer notes, HIV outreach workers and researchers include reasons why phone calls were the most frequent mode of communication, examples of how cell phone company text messages are used as a form of behavior change, literacy as a persistent barrier for some FEWs, and FEWs' high interest in receiving health-related messages and less concern about privacy and phone-sharing issues than expected. CONCLUSIONS: This study suggests texting is a part of normal phone use although not as frequently used as voice calls or Facebook. Despite the less frequent use, FEWs were able to send and receive messages, were interested in health messages and were not overly concerned about privacy issues. Texting and voice messaging may be useful tools for health behavior change within the FEW population in Cambodia.

Acceptability Study on HIV Self-Testing among Transgender Women, Men who Have Sex with Men, and Female Entertainment Workers in Cambodia: A Qualitative Analysis.

BACKGROUND: In Cambodia, HIV prevalence is high while HIV testing rates remain low among transgender women (TG women), men who have sex with men (MSM), and female entertainment workers (FEW). Introducing self-testing for HIV to these key populations (KPs) could potentially overcome the under-diagnosis of HIV and significantly increase testing rates and receipt of the results, and thus could decrease transmission. Therefore, this study aimed to determine the acceptability of HIV self-testing (HIVST) among these three categories of KPs. METHODS: This study was conducted through focus group discussions (FGDs) with TG women, MSM, and FEW in Phnom Penh city, Kampong Cham, Battambang, and Siem Reap provinces of Cambodia. Convenience sampling was used to recruit the participants. Two FGDs (six participants in each FGD) were conducted in each target group in each study site, totaling 24 FGDs (144 participants). Thematic analysis was performed to identify common or divergent patterns across the target groups. RESULTS: Almost all participants among the three groups (TG women, MSM, and FEW) had not heard about HIVST, but all of them expressed willingness to try it. They perceived HIVST as confidential, convenient, time-saving, and high-tech. Barriers to obtaining HIVST included cost, access, administration technique, embarrassment, and fear of pain. The majority preferred counseling before and after testing. CONCLUSIONS: Participants showed high willingness to use and acceptability of HIVST due to its confidentiality/privacy and convenience even if it is not linked to a confirmatory test or care and treatment. Notwithstanding, to increase HIVST, the target groups would need affordable self-test kits, education about how to perform HIVST and read results, assurance about accuracy and reliability of HIVST, and provision of post-test counseling and facilitation of linkage to care and treatment.