Project Abstract:

Magnetic memories (MRAM) and memristors are amongst the most promising technologies for emerging nonvolatile memories. MRAM implement concepts developed within spintronics, which uses spin –rather than electrons– to transfer and store information. In this project we will explore hybrid spintronic-memristor devices in graphene-based heterostructures comprising 2D transition metal dichalcogenides (TMDs) and less explored group-IV monochalcogenides (IV-MCs) materials. We will perform the first ever evaluation of the potential of 2D IV-MCs as memristors and implement graphene-based heterostructures with enhanced spin-orbit coupling using both TMDs and IV-MCs. With these heterostructures we aim at controlling graphene’s spin properties by changing the memristive setting of the chalcogenides. They will be made and characterized such that new multifunctional 2D systems are generated for applications in ultradense and ultralow power nonvolatile memories and neuromorphic computer architectures.

Project Objectives:

The overarching goal of this project is to nurture a paradigm shift by developing a new generation of two dimensional functional materials (2DFM) and heterostructures that can provide a leap towards novel computing architecture technologies with potential for ultra-low power consumption, and faster and cheaper information storage. To achieve this goal, the project will initiate a new research line that feeds from two active areas in condensed matter physics, namely spintronics and memristor systems.

Team:

The project team consists of:

1. Dr. Alin Velea - Project leader
2. Dr. Petre Bădică - Scientific researcher I
3. Dr. Mihail Secu - Scientific researcher I
4. Dr. Aurelian-Cătălin Gâlcă - Scientific researcher I
5. Dr. Florinel Sava - Scientific researcher II
6. Dr. Iosif-Daniel Șimăndan - Scientific researcher
7. Dr. Oana-Claudia Mihai - Research assistant
8. Dr. Angel-Theodor Buruiană - Research assistant

Stage 1: Tests for the development of functional 2D materials

Deadline: 31.12.2019

This first stage of the project aimed to perform tests for the development of functional 2D materials. In order to reach this objective, several activities have been carried out.

Tests to determine the optimum conditions for obtaining crystalline thin layers of SnSe were performed using the physical vapor transport method (PVT). The PVT method involves the transformation of the SnSe material from the condensed state (powder) into a vapor state, the transport of the vapours by the flow of argon into a quartz tube arranged horizontally and then condensation on a suitable support  (positioned inside the quartz tube), as a thin crystalline layer of SnSe.

The most important parameters in the process of deposition of thin SnSe layers by PVT are the temperature of the SnSe powder and the temperature of the Si substrate, the temperature at which the thin layer is deposited, the argon flow during deposition, the time of the deposition and the residual oxygen concentration in the quartz tube during deposition.

The analysis  of the obtained layers was done in the context of the influence of the deposition parameters on the quality of the obtained films. The temperature of the SnSe powder and Si substrate is an important parameter, inducing significant differences in the morphology of the obtained films. The second important parameter is the flow of argon during the deposition of the thin layers. Thus, it is found that to obtain a uniform deposition, the flow must be moderate, if it is greatly enlarged then the clusters are crowded towards the edge of the Si substrate. The third important parameter is the concentration of residual oxygen. The presence of oxygen in high concentration leads to oxidation of Tin vapors and their condensation on the "hot" substrate in the form of polycrystalline SnO2 clusters. If the residual oxygen concentration decreases, then only a part of the Sn vapors are oxidized, the rest, are condensing on the substrate as polycrystalline semispherical clusters of Sn. This difference in morphology is due to the difference in the melting temperature of Sn (231.93 °C) and SnO2 (1630 °C). If oxygen is absent, then on the surface of the substrate are formed Sn polycrystalline semispherical clusters. The Selenium vapors that are formed during deposition are partly evacuated, and another part is condensed as amorphous Selenium (melting temperature 221 °C). These results were obtained using X-ray diffraction (XRD) measurements, scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDX) and Raman spectrometry.

In conclusion, following the tests performed, valuable information was obtained which led to the identification of the most important deposition parameters and indications about the optimum values ​​for the preparation of polycrystalline layers of SnSe. In the next step, we will improve the preparation process to have a single polycrystalline phase (SnSe) on the substrate. We will also extend the preparation process to other group IV monochalcogenides.

Stage 2: Group IV monochalcogenides synthesis

Deadline: 31.12.2020

The second stage of the 2D-SPIN-MEM project aimed at obtaining 2D chalcogenide materials that have the chemical formula SX, S being an element of group IV (Ge or Sn) and X a chalcogen (S or Se). These materials in the literature are called group-IV monochalcogenides.

To achieve this goal, several activities were carried out in this second stage for the growth by CVD of group-IV monochalcogenides and the development of synthetic recipes. First, the method of obtaining was innovated on the basis of the experience gained in the first stage. The deposition parameters were varied to identify the optimal values in order to achieve the proposed objective: a high density of monocalcogenide flakes with the dimensions: length × width > 5 × 5 μm. 25 syntheses were carried out using SnSe monocalcogenide resulting in 25 samples. Following the investigation of the quality and density of SnSe flakes, an optimized preparation recipe resulted. This was followed by an etching process resulting in 2D SnSe single/few-layer crystals. This process was also optimized to obtain 2D flat SnSe crystals, without etching residues. These etched flakes are suitable for transfer to graphene. The transfer and proximity effects studies will be carried out in the next stage of the project. Next, we proceeded to CVD synthesis tests of group-IV monochalcogenides doped with transition metals, namely obtaining Cr-doped SnSe flakes (a transition metal) by modifying the synthesis recipe.

The quality of the obtained flakes was evaluated and the detailed physico-chemical characterization of the best 2D samples was performed. Optical microscopy has shown that the abundance of flakes is optimal for achieving the goals of this project. Using scanning electron microscopy (SEM) we investigated the quality of the SnSe flakes produced. The edges of the flakes were found to be smooth and well defined. Using energy dispersive X-ray spectroscopy (EDX), the chemical composition of the flakes was measured.

Detailed information on the crystal structure of a SnSe flake was obtained by transmission electron microscopy (TEM) and EDX. The electron diffraction (SAED) figure made at the edge of a SnSe flake (where the flake thickness is only a few atomic layers of SnSe) shows an orthogonal symmetry, and the elementary cell constants are very close to the theoretical values for SnSe. Mapping of Sn and Se elements in another flake indicates that the Sn and Se atoms are evenly distributed. EDX measurements were performed in three areas of the flake, which indicate an atomic ratio Sn:Se very close to 50%: 50%.

Raman spectroscopy was performed to provide more information about the structure quality of SnSe flakes. Ag and B3g are two rigid shear modes of a layer relative to its neighbors and are the characteristic modes of flat vibration in SnSe. For a monocrystalline SnSe flake, four maxima are clearly observed at 70.2 cm-1, 107.35 cm-1, 129.85 cm-1 and 150.54 cm-1. The Raman maximum from 107.35 cm-1 corresponds to the B3g vibration mode, while the other three observed maxima belong to the Ag vibration mode.

Tests were performed for the development and construction of memristive devices for further evaluation. To build memristive devices based on SnSe flakes, a process was used which consisted in growing SnSe flakes on a silicon support, by PVT method, and then transferring at least one SnSe flake on comb-type electrical contacts located on another substrate. The I-V characteristics of the obtained devices showed a Schottky-type bipolar behavior.

The best 2D flakes produced will be used to study the effects of proximity in graphene. These were sent to Spain to the ICN2 Partner (Catalan Institute of Nanoscience and Nanotechnology). These flakes are to be transferred to graphene and the proximity effects to be studied.

In conclusion, rectangular and square flakes of SnSe and Cr-doped SnSe, with average dimensions between 5 and 30 μm, were successfully obtained using a CVD method. These flakes were corroded in a nitrogen atmosphere at atmospheric pressure, their thickness being reduced to one or few atomic layers.

Morphology, microstructure and chemical composition of SnSe and Cr: SnSe flakes were characterized by light microscopy, SEM, EDX, TEM and Raman.

The first SnSe-based devices were produced and their memristive properties were investigated.

Given the growing interest in group IV monochalcogenides, this simple two-step synthesis technique can be used to obtain other 2D materials from the same family, for applications in electronics, optoelectronics, sensors or energy conversion.

The objectives of this research stage were fully achieved.

Simple and clean method for obtaining Sn nanoparticles for hydrophobic coatings
A.T. Buruiana, F. Sava, E. Matei, I. Zgura, M. Burdusel, C. Mihai, A. Velea
Materials Letters 278, 128419 (2020) 
doi: https://doi.org/10.1016/j.matlet.2020.128419

Project Contact Person:

Dr. Alin Velea

alin.velea@infim.ro