Bismuth telluride (Bi 2 Te 3 ) and its alloys are some of the best available materials for near-room-temperature thermoelectric applications. [ 1 ] In particular, Bi 2 Te 3 nanowires have been studied extensively [3][4][5] because low-dimensional thermoelectric materials are expected to have a higher fi gure of merit due to quantum confi nement effects. [ 2 ] However, memory switching behavior has never been studied in Bi 2 Te 3 nanowires. Here, we report for the fi rst time reversible memory switching effects in Bi 2 Te 3 nanowires fabricated using anodized aluminum oxide (AAO) membranes. The fi ndings show that Bi 2 Te 3 nanowires display a reversible crystalline-amorphous phase change that is induced by a temperature, laser, or electric fi eld, similar to that reported for chalcogenide materials (Ge-Sb-Te alloys, GST). [6][7][8][9] We demonstrate that Bi 2 Te 3 nanowires show considerable promise as building blocks for phase-change random access memory (PRAM).Phase-change materials are used in nonvolatile optical memory (e.g., CDs and DVDs), and are being actively investigated as the media in universal solid-state memory devices that combine rapid read and write speeds, high storage density, and non-volatility. [ 10 ] The key feature of PRAM is the reversible phase transition of the phase-change material, caused by an electrical pulse, between the crystalline (low resistivity, SET) and amorphous (high resistivity, RESET) states.A major obstacle to achieving high-density PRAM devices is the large writing currents required to generate suffi cient thermal energy for a phase change, particularly during the crystal-to-amorphous phase transition, since a high current is required for melting. To reduce the writing currents, GST nanowires have been synthesized and were shown to satisfy many of the attributes of universal non-volatile memory devices. [ 12 , 13 ] However, GST nanowires are usually synthesized using vapor transport methods at high temperatures [11][12][13] and their large-scale assembly is not yet feasible. On the other hand, Bi 2 Te 3 nanowires exhibit memory switching characteristics that are comparable to GST nanowires [ 11 , 12 ] and they can be fabricated at room temperature using AAO membranes.Furthermore, the vertical growth of nanowires on a substrate permits a high-density assembly of Bi 2 Te 3 nanowires. Here, we describe in detail the memory switching properties of Bi 2 Te 3 nanowires.We fabricated Bi 2 Te 3 nanowires using electrodeposition within the nanopores of an AAO membrane made by anodizing Al plates. The nanowire structures were examined using X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). An XRD pattern of the as-grown nanowires ( Figure 1 a) agreed with the rhombohedral crystal structure of Bi 2 Te 3 (JCPDS No. 15-0863) reported by others. [ 3 , 4 ] The spacing between adjacent planes in the HRTEM image (Figure 1 b) was 0.202 nm, which corresponded to the (110) lattice planes of the rhombohedral Bi 2 Te 3 crystal structure. In addition, the...
The wettability of graphene has been extensively studied and successfully modified by chemical functionalization. Nevertheless, the unavoidable introduction of undesired defects and the absence of systematic and local control over wettability by previous methods have limited the use of graphene in applications. In addition, microscale patterning, according to wettability, has not been attempted. Here, we demonstrate that the wettability of graphene can be systematically controlled and surface patterned into microscale sections based on wettability without creating significant defects, possible by nondestructive hydrogen plasma. Hydrophobic graphene is progressively converted to hydrophilic hydrogenated graphene (H-Gr) that reaches superhydrophilicity. The great contrast in wettability between graphene and H-Gr makes it possible to selectively position and isolate human breast cancer cells on arrays of micropatterns since strong hydrophilicity facilitates the adsorption of the cells. We believe that our method will provide an essential technique for enabling surface and biological applications requiring microscale patterns with different wettability.
Neural stem cells (NSCs) are characterized by a capacity for self-renewal, differentiation into multiple neural lineages, all of which are considered to be promising components for neural regeneration. However, for cell-replacement therapies, it is essential to monitor the process of in vitro NSC differentiation and identify differentiated cell phenotypes. We report a real-time and label-free method that uses a capacitance sensor array to monitor the differentiation of human fetal brain-derived NSCs (hNSCs) and to identify the fates of differentiated cells. When hNSCs were placed under proliferation or differentiation conditions in five media, proliferating and differentiating hNSCs exhibited different frequency and time dependences of capacitance, indicating that the proliferation and differentiation status of hNSCs may be discriminated in real-time using our capacitance sensor. In addition, comparison between real-time capacitance and time-lapse optical images revealed that neuronal and astroglial differentiation of hNSCs may be identified in real-time without cell labeling.
We investigated the memristive switching behavior in bismuth-antimony alloy (Bi(1-x)Sb(x)) single nanowire devices at 0.1 ≤ x ≤ 0.42. At 0.15 ≤ x ≤ 0.42, most Bi(1-x)Sb(x) single nanowire devices exhibited bipolar resistive switching (RS) behavior with on/off ratios of approximately 10(4) and narrow variations in switching parameters. Moreover, the resistance values in the low-resistance state (LRS) were insensitive to x. On the other hand, at 0.1 ≤ x ≤ 0.15, some Bi(1-x)Sb(x) single nanowire devices showed complementary RS-like behavior, which was ascribed to asymmetric contact properties. Transmission electron microscopy and elemental mapping images of Bi, Sb, and O obtained from the cross sections of the Bi(1-x)Sb(x) single nanowire devices, which were cut before and after RS, revealed that the mobile species was Sb ions, and the migration of the Sb ions to the nanowire surface brought the switch to LRS. In addition, we demonstrated that two types of synaptic plasticity, namely, short-term plasticity and long-term potentiation, could be implemented in Bi(1-x)Sb(x) nanowires by applying a sequence of voltage pulses with different repetition intervals.
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