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There is an increasing importance of memory technologies in our ever-digitalizing society, which is characterized by the generation and use of a tremendous amount of real-time data. Beyond traditional performance requirements, mechanical flexibility of memory systems becomes therefore critical to enabling emerging applications such as the Internet of things. Graphene, now an established nanomaterial platform, is a promising element for building such high-performance memories with an unconventional form factor because of its exceptional electrical conductivities, outstanding mechanical properties, and processing versatility desirable for hybrid integration. Here, we provide an overview of recently developed flexible memory devices based on graphene and its functionalized counterparts. A defining feature of this review is that it exclusively compares and analyzes the devices that meet the following two criteria: (i) an explicit demonstration of working devices on a flexible substrate is reported; (ii) graphene is employed as an active channel rather than as an electrode. Our primary focus is to systematically classify various types of memories, in view of their materials, structural, and functional characteristics. For this, the shapes and compositions of the conductive channel, the key operational mechanisms underlying the electrical functionalities, and the major characteristics of representative device structures and materials systems are carefully evaluated. Furthermore, the applicability of flexible graphene memories to the neuromorphic computing area is discussed. Finally, we address several remaining issues that need to be solved for future technological advancements.

Organic semiconductors are highly interface-sensitive, and therefore chemical functionalization using self-assembled monolayers (SAMs) is often adopted to tailor their properties. This study clarifies the synergistic effects of electrode and dielectric SAMs on the behavior of solution-processed organic field-effect transistors (OFETs). Utilization of a self-consistent device model enables a physically robust treatment of the measured electrical characteristics of the OFETs, thus providing highly reliable materials, interface, morphology, and transport parameters. These parameters are further extended and correlated to build a comprehensive picture on trap energy and injection-transport relationship, finally revealing a set of fundamental insights into chemically modified OFETs.

Within the approach based on chemical modification of domain boundaries of polycrystalline graphene, a transistor channel is proposed for enhanced current modulation, Ion/Ioff ratio, in the 3 to 5 order of magnitude. We observed that two types of samples functionalized by N-methylpirrolidone (NMP) and weakly fluorinated graphene are able to demonstrate high current modulation. Experimentally, Ion/Ioff ~ 103 was found for NMP functionalized graphene and Ion/Ioff ~ 104–105 for weakly fluorinated graphene. Modeling of these two systems allows us to clarify the mechanism of carrier transport in the multi-barrier films of functionalized graphene films. It is shown that remarkable value for Ion/Ioff as about 106 can be observed for the films comprising graphene regions (graphene quantum dots, GQDs) with size of ~ 30–300 nm and ~ 75–100 nm fluorinated graphene barriers. Relatively high values of Ion/Ioff ~ 103–105 are also predicted for large graphene areas separated with thin (100 nm) barriers for the weakly fluorinated graphene samples. Our study paves a way towards controllable 2D transistors.

In this paper, we proposed optical filter based on two-dimensional photonic crystal elliptical channel for wavelength division multiplexing. We employed ring resonator with ellipse shaped core in proposed structure. The resonance wavelength of the ring resonators depends on two parameters: refractive index and radius of dielectric rods. We investigate the physical parameters governing the filter performance. Our results show that the transmission efficiency is 100 % due to nearly zero reflection and loss in channel. Band width of 4 nm and quality factor of 389 is obtained for filter. Also, the total footprint of proposed structure is 230.4 µm2 which makes it suitable for all optical integrated circuits.

In this paper, we propose a high-quality factor (QF) ultra-compact structure for designing all optical filter based on two dimensional (2D) photonic crystals that can operate in third communication window range (1550 nm). The simulation results calculated by using 2D finite difference time domain (FDTD) method and photonic band gap (PBG) region are calculated by plane wave expansion (PWE) method. The average transmission efficiency, average QF and optical full width at half maximum (FWHM) bandwidth more than 95 %, 4300 and 0.4 nm obtained, respectively. The value of QF for the proposed structure is a major QF for ring resonator-based filters. We studied the effect of changing the air background to liquid crystal (LC) surrounding the dielectric rods. Results showed that by increasing LC refractive index (n) peak of wavelengths red shifted. The footprint of proposed filter is 160 ${{\mu}}{{\rm{m}}^{\rm{2}}}$ which make the proposed structure promising candidate for optical integrated circuits.

Transport properties of biased planar Gaussian graphene superlattice (PGGSL) with Fermi velocity barrier is investigated by transfer matrix method (TMM). It is observed that enlargement of bias voltage over miniband width breaks the miniband to WSLs leads to suppressing resonant tunneling. Transmission spectrum shows flat wide stop-band property controllable by external bias voltage with stop-band width of near 200 meV. The simulations demonstrate that strong velocity barriers prevent tunneling of Dirac electrons leading to controllable enhancement of stop-band width. By increasing ratio of Fermi velocity in barriers to wells υ c stop-band width increase. As wide transmission stop-band width (BWT) of filter is tunable from 40 meV to 340 meV is obtained by enhancing ratio of υ c from 0.2 to 1.5, respectively. Proposed structure suggests easy tunable wide band-stop electronic filter with a modulated flat stop-band characteristic by height of electrostatic barrier and structural parameters. Robust sensitivity of band width to velocity barrier intensity in certain bias voltages and flat band feature of proposed filter may be opens novel venue in GSL based flat band low noise filters and velocity modulation devices.

We have proposed simple ring resonator 5-channel demultiplexer based on optical channel drop filter analysis that is applicable at third communication window (1550 nm) range. Our proposed base filter is the important part in designing the demultiplexer, inclusive one ring resonator contains one square dielectric rod at core. Demultiplexer structure introduced by arranging five filter with different ring core refractive index. Insomuch every ring core have individual refractive index, thus each ring have diverse resonant wavelength. Numerical results by the finite difference time domain (FDTD) method show quality factor (Q) and transmission efficiency of fundamental channel drop filter are 1038 and 93 %, respectively. It is found that transmission efficiency in designed demultiplexer is more than 90 % for each channel; channel spacing is less than 4.2 nm. The average crosstalk value, total footprint of demutiplexer is −17.85 dB, 689.61 μm2, respectively. Small size with a very simple ring design can be benefit in photonic integrated circuit.

In this study, we successfully demonstrate the growth of an in-plane graphene/h-BN (GBN) heterostructure on a single crystal Ge (1 1 0) substrate. A group IV semiconductor Ge is an appropriate catalyst for the epitaxial growth of both graphene and h-BN. Thus, by sequentially introducing ammonia borane (NH3-BH3) and methane using two-zone low-pressure chemical vapor deposition (LPCVD), we obtained an in-plane GBN heterostructure. Based on microscopic and spectroscopic analyses, we confirmed that the edge of the pre-synthesized h-BN domains provides plentiful nucleating sites for the lateral epitaxial growth of graphene. Furthermore, we systematically controlled the area and density of h-BN domains in GBN and observed a change in the electrical conductivity of GBN based on the ratio of conducting graphene and insulating h-BN. This result conforms to the percolation theory of two-dimensional materials (2DMs). We believe that our synthetic approach could be a practical method for large-scale synthesis and property control of in-plane heterostructures and can be applied to various types of 2D heterostructures—potentially useful in a wide range of electronic applications.

Here, we proposed a high performance 16-channel optical demultiplexer using two-dimensional photonic crystal ring resonator for telecommunication systems. By plane wave expansion (PWE) method the photonic band gap (PBG) of proposed structure calculated. Then, with finite difference time domain (FDTD) method the performance parameters of designed two-dimensional photonic crystal demultiplexer are analyzed. It is found that the channel wavelength of wavelength-division multiplexing (WDM) is truly tuned by changing the structure parameters of the demultiplexer and position of rod. Output peaks located in the optical communication C-band and L-band with the transmission efficiency of 99 %. The demultiplexer exhibits high-quality factor of 5176, and spectral width of 0.3. Very low crosstalk values are between −19 dB and −90 dB where, device only occupies an area of 1708.65 µm2. The proposed compact 16-channel demultiplexer can find more applications for the ultra-compact WDM systems in highly integrated telecommunication circuits.

Here, we present a detailed study on low bias current-voltage (I-V) characteristic of graphene superlattice (GSL) resonant tunneling diode (RTD) with heterostructured substrate and series of grounded metallic planes placed over graphene sheet which induce periodically modulated Dirac gap and Fermi velocity barrier. We investigate the effect of GSL parameters on I-V characteristics within the Landauer-Buttiker formalism and adopted transfer matrix method. We show how the engineering these parameters results in multipeak NDR in proposed device. Moreover we provide a novel venue to control the NDR in GSL with Fermi velocity engineering. From this viewpoint we obtain multipeak NDR through miniband align in GSL. Maximum Pick to Valley ratio (PVR) up to 167 obtained for correlation velocity of 1.9 and bias voltages between 70-130 mV. Our finding in low bias regime and high PVR multipeak NDR have a considerable importance in multi-valued memory functional circuit, low power and high-speed nanoelectronic devices application., 13 pages, 5 Figures

Negative differential resistance (NDR) in two dimensional materials has been the subject of strong interest for ultra-low power nanoelectronic applications. Here, we report NDR characteristics of silicene superlattice (SL) at low bias voltages. Transport process manipulated by miniband regime for low bias region and Wannier-Stark (WS) ladders regime with multi-peak NDR for higher bias windows. Local tunneling peaks rise from hybridization of Wannier-Stark rungs in certain bias voltages. The bias position of WS states crossings down shifted with increasing device dimension leading to red shifted NDR window and increases peak to valley ratio (PVR) values. The multiple NDR windows are represented by changing the size and the number of well/barriers in silicene SL. Maximum PVR of 8 obtained for device with low height barriers. Multi-peak NDR with same PVR values in very low bias regime can find key applications in multi-valued memories with low static power dissipation.

Partially fluorinated graphene films were created by chemical functionalization of graphene layers in an aqueous solution of hydrofluoric acid. The formation of graphene islands or graphene quantum dots (GQDs) and a fluorinated graphene network is demonstrated in such films. Negative differential resistance (NDR) resulting from the formation of the potential barrier system in the films was observed for different fluorination degrees of suspension. The origin of the NDR varies with an increase in the fluorination degree of the suspension. Numerical calculations were performed to elucidate the tunneling between adjacent energy levels and creation of NDR. It was found that in the case of films with smaller flake and smaller GQD sizes, multi-peak NDR appears in the I–V curve. We predict that the NDR peak position shifts towards lower voltage with a decrease in the GQD size. Surprisingly, we observed a negative step-like valley for positive biases in the I-V curve of samples. Our findings with detailed analysi...

Here, we study the negative differential resistance (NDR) of Dirac electrons in biased planar graphene superlattice (PGSL) and investigate the transport characteristics by adopted transfer matrix method within Landauer-Buttiker formalism. Our model device is based on one-dimensional Kronig–Penney type electrostatic potential in monolayer graphene deposited on a substrate, where the bias voltage is applied by two electrodes in the left and right. At Low bias voltages, we found that NDR appears due to breaking of minibands to Wannier-Stark ladders (WSLs). At the critical bias voltage, delocalization appeared by WS states leads to tunneling peak current in current-voltage (I-V) characteristics. With increasing bias voltage, crossing of rungs from various WSL results in multi-peak NDR. The results demonstrate that the structure parameters like barrier/well thickness and barrier height have remarkable effect on I-V characteristics of PGSL. In addition, Dirac gap enhances peak to valley (PVR) value due to suppr...