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Ministry of Science and Technology of China (grant nos. 2019YFE0124200 and 2018YFE0100800), National Natural Science Foundation of China (grant no. 61874075), Baseline funding scheme of the King Abdullah University of Science and Technology

Nanoscale thermal sensing of devices with calibrated scanning thermal microscopy. The calibration allows converting the electrical response of thermo-resistive probes into temperature and considers variable probe-sample thermal exchange processes.

En el presente ensayo se presentan algunas reflexiones sobre el concepto de aprendizaje y los efectos que ha tenido la pandemia por el Covid-19 iniciada en el año 2020 en relación con el contexto educativo, específicamente, en el proceso de aprendizaje de los estudiantes del grado tercero de la Institución Educativa Presbítero Antonio José Bernal Londoño, así como algunas de las estrategias utilizadas tanto en la institución, como por las educadoras del grado, para contrarrestar los efectos negativos en las emociones de los estudiantes y en los vacíos académicos, evidenciados en los procesos de lectura, escritura y resolución de problemas.

The increasing threat of multidrug-resistant microorganisms is a cause of worldwide concern. This motivates a necessity to discover new antimicrobial agents or new mechanisms for microorganism eradication, different from those currently used. Here, we report an effective antibacterial ceramic glaze that combines different bactericidal mechanisms. Specifically, the used methodology of the glaze results in glass-free edge crystallizations of feldspar structures at the ceramic surface. A combination of Rutherford backscattering spectroscopy, scanning electron microscopy, and Raman microscopy is used to determine the chemical elements and crystallizations at the ceramic surface. Moreover, Kelvin probe force microscopy demonstrates that the presence of glass-free edges in feldspar needle crystals (semiconductor phase) on a glass matrix (insulator phase) promotes the formation of semiconductor-insulator interface barriers. These barriers act as reservoirs of electric charges of ∼1.5 V, producing a discharge exceeding the microorganism membrane breakdown value (up to 0.5 V). Furthermore, the surface crystallizations account for the formation of a microroughness that limits biofilm formation. Both factors result in high antibacterial activity in the range of R > 4 for Escherichia coli and Staphylococcus aureus. This approach opens new possibilities to attain bactericidal surfaces and to understand the role of physical interaction as a main antimicrobial mechanism., The authors acknowledege the financial support by the projects MAT2017-86450-C4-1-R and MAT2017-86450-C4-3-R.

Tras más de medio siglo de las independencias de los países africanos gran parte de sus Estados siguen siendo cuestionados por parte de la propia población africana, de organismos internacionales y de distintos países. Diferentes posturas teóricas vienen a explicar lo que denominan como “el fracaso de los Estados africanos” ya sea señalando causas externas o internas a los diferentes países africanos. Teniendo en cuenta los distintos análisis, este artículo pretende –evitando los reduccionismos, el racismo académico y desde una perspectiva histórica crítica– poner en cuestión el modelo de Estado que se ha pretendido establecer tras los procesos de independencia, al no reconocerse estos como modelos creados por y para los africanos. Palabras clave: Estado; colonialismo; democracia; clientelismo; África; poder.

Trabajo presentado en la International 16th Conference on Nanostructured Materials, NANO 2022, celebrada en Sevilla (España), del 6 al 10 de junio de 2022, Resistive random-access memories (RRAM) hold promise for developing future information technologies with ultra-high storage densities to process unprecedented amounts of data. RRAM operation relies on the formation (set) and rupture (reset) of nanoscale conductive filaments (CFs) with diameters as low as few nanometers in the switching layer. These filaments carry enormous power densities (>1013 W/cm3) that result in high localized temperatures. The heating behavior of these filaments lies at the heart of this technology but little is known experimentally about it. Understanding the temperature of nanoscale filamentary hot spots is key to developing more energy-efficient devices and avoiding thermal cross-talk in future dense RRAM arrays. In this work we report the first thermal measurements of single filament switching in RRAM. For that purpose, we use calibrated scanning thermal microscopy (SThM). This technique allows thermal metrology at sub-50 nm dimensions. We use SThM to locate such filaments in HfO2 RRAM devices with nanoscale resolution and determine the hot spot temperature at the top surface during operation. Then, we match the temperature profile with finite element simulations, using the filament diameter and the thermal boundary conductance at the filament-top electrode interface as fitting parameters, assuming diffusive transport in the filament. We use both conventional (metal) and novel (graphene) top electrodes to assess the effect of heat spreading in memory devices. Our study reveals that a CF of 4 nm with TiN top electrode and 13 nm with single layer graphene as top electrode at power ~100 ¿W can lead to a temperature rise as high as ~1100 K above ambient temperature. We solve a fundamental yet elusive problem that has existed in the data storage community for two decades, i.e. imaging the spatial extent and temperature of the filament operation in metal-oxide-based RRAM. Based on the information provided by the SThM measurement, thermal engineering approaches could be applied to develop energy efficient devices. As an example, one could choose electrodes with a thermal boundary conductance that is low at the filament electrode interface to confine the CF heating, and high at the surrounding oxide interfaces to minimize the lateral heat spreading. Overall, these results advance our knowledge of RRAM engineering for digital storage vs analog computing, our understanding of breakdown in insulators, and showcase a unique application of the SThM technique with ramifications much beyond memory technology.

Trabajo presentado en la International 16th Conference on Nanostructured Materials, NANO 2022, celebrada en Sevilla (España), del 6 al 10 de junio de 2022, Vanadium dioxide (VO2) is widely studied for its prominent insulator-metal transition (IMT) near room temperature, with potential applications in novel memory devices and brain-inspired neuromorphic computing. In previous work of our group, Rana et al. observed multiple intermediate stable resistive states between the insulating and metallic states in VO2 films by tailored temperature sweeps or external electrical stimuli1 . The existence of these intermediate resistive states is particularly attractive for reconfigurable electronic circuitry. In this work, we fabricated planar bridge-structure devices from VO2 thin films, which were grown on TiO2 (001) substrates by pulsed laser deposition (PLD). Traditionally, under voltage/current sweeps, Joule heating in the device triggers the IMT. The switching power depends on the device dimensions. However, in our devices, intermediate steps can happen during the reset when tuning the voltage under a high compliance current. This unique measurements can allow multistate memory within one VO2-based memory cell (in our demonstration 3 bits per cell) and reliable multilevel operation 2 . In order to exploit these intermediate states, we also fabricated devices with two identical parallel VO2 bridges with varying bridge-to-bridge distances. Depending on the bridge distance, we obtain a higher degree of control for the intermediate states. The switching behavior of these bridges depend ultimately on heat dissipation effects. To develop optimum device designs, we carried out nanoscale thermal mapping of in operando devices with varying bridgeto-bridge distances using Scanning Thermal Microscopy (SThM). The concept can readily be extended to more parallel channels and complex network configurations. Moreover, one may further use multilayering to stack bridges in the vertical direction and provide a further dimension for multibit operations

Resumen del trabajo presentado en la 16th International Conference on Nanostructured Materials NANO 2022, celebrada en Sevilla (España), del 6 al 10 de junio de 2022, Self-heating and localized temperatures play an important role in the principle of operation of nano- and micro-scale devices. On the one hand, heating in transistor devices affects the mobility of the carriers, limiting device performance and lifetime. On the other hand, energy dissipation in memory devices is connected to some drawbacks, like reliability and energy efficiency. Understanding the energy dissipation mechanisms is therefore essential for the evaluation, design and optimization of our electronic devices. Optical techniques, like infrared (IR) or Raman thermometry, can be used to obtain thermal maps of devices but their spatial resolution is diffraction limited, i.e., ~5 µm and ~0.5 µm respectively. Scanning thermal microscopy (SThM) is a scanning probe microscopy technique that allows thermal maps of devices with nanoscale resolution (~50 nm). Therefore, SThM is a promising tool for determining local hot spots and self-heating of different types of devices. In this work, we show how SThM can be employed for the characterization of heat dissipation in nanoelectronics. Our SThM uses a thermo-resistive probe whose electrical resistance varies with temperature. This probe can be used as a nanoscale sensor to map the temperature of devices locally. First, we present challenges associated with the calibration of this probe, which are key to obtaining quantitative measurements of device temperatures. Second, we show how a calibrated SThM system can be used to gain knowledge of the energy dissipation of memory devices. We focus on resistive random access (RRAM) and phase change (PCM) memory devices, which show promise for applications such as non-volatile memory and neuromorphic computing. The SThM thermal maps show the filamentary heating from RRAM devices as well as the Joule heated PCM device, displaying local temperature features. These maps provide insights into device operation, showing how the energy dissipates and offering new routes for developing more efficient switching mechanisms

Trabajo presentado como Seminar at the Institute of Micro and Nanotechnology. Disponible en Youtube: https://www.youtube.com/watch?v=UGtTr4E1JR8, One of the greatest challenges of modern society is related to energy consumption, dissipation and waste. A prominent example is that of integrated electronics, where power dissipation issues have limited their performance. In the first part of my talk, I will discuss energy dissipation in electronics, like in transistors based on 2D materials or resistive random-access memory (RRAM). On the one hand, the use of 2D materials, like MoS2, represent new opportunities for the electronic industry. However, understanding their thermal properties, like the thermal conductivity or thermal boundary resistance, is essential to achieve effective energy dissipation and avoid limited performance due to overheating. On the other hand, knowing the temperature reached by RRAM filaments is key to improve and design more efficient memory devices. However, its experimental characterization has been a long lasting challenge due to the localized heating of the filament at the nanoscale. Additionally, advanced management of the heat that is dissipated by these systems for energy conversion or storage might have a major technological impact. In the second part of my talk, I will present thermal devices that are capable to manage heat in a manner analogous to how electronic devices control electricity. I will give an example of a thermal diode that can be applied for energy storage applications.

Resistive random access memory (RRAM) is an important candidate for both digital, high-density data storage and for analog, neuromorphic computing. RRAM operation relies on the formation and rupture of nanoscale conductive filaments that carry enormous current densities and whose behavior lies at the heart of this technology. Here, we directly measure the temperature of these filaments in realistic RRAM with nanoscale resolution using scanning thermal microscopy. We use both conventional metal and ultrathin graphene electrodes, which enable the most thermally intimate measurement to date. Filaments can reach 1300°C during steady-state operation, but electrode temperatures seldom exceed 350°C because of thermal interface resistance. These results reveal the importance of thermal engineering for nanoscale RRAM toward ultradense data storage or neuromorphic operation., This work was supported in part by ASCENT, one of six centers in JUMP, a Semiconductor Research Corporation (SRC) program sponsored by DARPA.

Two-dimensional (2D) materials have recently been incorporated into resistive memory devices because of their atomically thin nature, but their switching mechanism is not yet well understood. Here we study bipolar switching in MoTe2-based resistive memory of varying thickness and electrode area. Using scanning thermal microscopy (SThM), we map the surface temperature of the devices under bias, revealing clear evidence of localized heating at conductive "plugs" formed during switching. The SThM measurements are correlated to electro-thermal simulations, yielding a range of plug diameters (250 to 350 nm) and temperatures at constant bias and during switching. Transmission electron microscopy images reveal these plugs result from atomic migration between electrodes, which is a thermally-activated process. However, the initial forming may be caused by defect generation or Te migration within the MoTe2. This study provides the first thermal and localized switching insights into the operation of such resistive memory and demonstrates a thermal microscopy technique that can be applied to a wide variety of traditional and emerging memory devices.

Vanadium dioxide (VO2) has been widely studied for its rich physics and potential applications, undergoing a prominent insulator-metal transition (IMT) near room temperature. The transition mechanism remains highly debated, and little is known about the IMT at nanoscale dimensions. To shed light on this problem, here we use ~1 nm wide carbon nanotube (CNT) heaters to trigger the IMT in VO2. Single metallic CNTs switch the adjacent VO2 at less than half the voltage and power required by control devices without a CNT, with switching power as low as ~85 ${\mu}W$ at 300 nm device lengths. We also obtain potential and temperature maps of devices during operation using Kelvin Probe Microscopy (KPM) and Scanning Thermal Microscopy (SThM). Comparing these with three-dimensional electrothermal simulations, we find that the local heating of the VO2 by the CNT play a key role in the IMT. These results demonstrate the ability to trigger IMT in VO2 using nanoscale heaters, and highlight the significance of thermal engineering to improve device behaviour.

Over the past decade, solid‐state thermal control devices have emerged as potential candidates for enhanced thermal management and storage. They distinguish themselves from traditional passive thermal management devices in that their thermal properties have sharp, nonlinear dependencies on direction and operating temperature, and can lead to more efficient circuits and energy conversion systems than what is possible today. They also distinguish themselves from traditional active thermal management devices (e.g., fans) in that they have no moving parts and are compact and reliable. In this article, the recent progress in the four broad categories of solid‐state thermal control devices that are under active research is reviewed: diodes, switches, regulators, and transistors. For each class of device, the operation principle, material choices, as well as metrics to compare and contrast performance are discussed. New architectures that are explored theoretically, but not experimentally demonstrated, are also discussed.

Although phase-change materials (PCMs) have been studied for more than 50 years, temperature-dependent characterization of the phase transition dynamics remains challenging due to the lack of nanosecond-nanoscale thermometry. In this article, we utilize the four-terminal, indirectly heated phase-change switch (IPCS), which was originally designed for nonvolatile radio frequency (RF) applications, as an ultrafast electrothermal platform to study PCM. We propose a novel experimental setup that allows nanosecond probing of the transient resistance of the PCM, beyond the melting temperature (>1100 K), due to the built-in electrical isolation between the PCM path and the thermal actuation path of the IPCS. The embedded metallic heater can induce reversible phase transitions between the crystalline and amorphous phases of the PCM. Our platform enables simultaneous measurements of the dynamics of PCM resistance (as a probe for the phase of the material) and heater temperature, during the application of heating pulses. Furthermore, we map the surface temperature of the IPCS at steady state by scanning thermal microscopy (SThM) and show the effect of cooling by electrodes in devices with overlap between the heater and PCM contacts. Our method can be used to study chalcogenides and other amorphous semiconductors for reconfigurable electronics and neuromorphic hardware.

In recent years, intensive studies on thermal control devices have been conducted for the thermal management of electronics and computers as well as for applications in energy conversion, chemistry, sensors, buildings, and outer space. Conventional cooling or heating techniques realized using traditional thermal resistors and capacitors cannot meet the thermal requirements of advanced systems. Therefore, new thermal control devices are being investigated to satisfy these requirements. These devices include thermal diodes, thermal switches, thermal regulators, and thermal transistors, all of which manage heat in a manner analogous to how electronic devices and circuits control electricity. To design or apply these novel devices as well as thermal control principles, this paper presents a systematic and comprehensive review of the state-of-the-art of fluidic and mechanical thermal control devices that have already been implemented in various applications for different size scales and temperature ranges. Operation principles, working parameters, and limitations are discussed and the most important features for a particular device are identified.