Letter pubs.acs.org/NanoLett
Crystalline−Crystalline Phase Transformation in Two-Dimensional In2Se3 Thin Layers Xin Tao and Yi Gu* Department of Physics and Astronomy, Washington State University, Pullman, Washington 99164, United States S Supporting Information *
ABSTRACT: We report, for the first time, the fabrication of single-crystal In2Se3 thin layers using mechanical exfoliation and studies of crystalline−crystalline (α → β) phase transformations as well as the corresponding changes of the electrical properties in these thin layers. Particularly, using electron microscopy and correlative in situ micro-Raman and electrical measurements, we show that, in contrast to bulk single crystals, the β phase can persist in single-crystal thin layers at room temperature (RT). The single-crystal nature of the layers before and after the phase transition allows for unambiguous determination of changes in the electrical resistivity. Specifically, the β phase has an electrical resistivity about 1−2 orders of magnitude lower than the α phase. Furthermore, we find that the temperature of the α → β phase transformation increases by as much as 130 K with the layer thickness decreasing from ∼87 nm to ∼4 nm. These single-crystal thin layers are ideal for studying the scaling behavior of the phase transformations and associated changes of the electrical properties. For these In2Se3 thin layers, the accessibility of the β phase at RT, with distinct electrical properties than the α phase, provides the basis for multilevel phase-change memories in a single material system. KEYWORDS: In2Se3, thin layers, phase transformation, mechanical exfoliation, Raman spectroscopy, transmission electron microscopy
T
the same lattice type but with slightly different lattice parameters (see Table 1). Upon cooling to RT, the α phase always reverts back to the β phase in bulk single-crystal In2Se3. In contrast, in In2Se3 polycrystalline powders, a mixture of α and β phases has been observed at RT.12 This indicates that the β phase might be stabilized in small crystallites, although whether the crystal size plays a role remains unclear. Moreover, the mixed phases and the polycrystallinity of powders prevent a unambiguous identification of phase-specific properties at RT, which are important for phase-change memory applications. Here we report, for the first time (to the best of our knowledge), the fabrication of single-crystal In2Se3 thin layers using mechanical exfoliation and studies of the α → β phase transformation as well as the corresponding changes in the electrical properties using electron microscopy, correlative in situ micro-Raman, and electrical measurements. Particularly, we show that, unlike bulk single crystals, the β phase can persist in single-crystal thin layers at RT. The β phase has an electrical resistivity about 1−2 orders of magnitude lower than the α phase. Furthermore, we find that the temperature of the α → β phase transformation increases as the layer thickness decreases. The accessibility of the β phase at RT, with distinct electrical properties than the α phase, provides the basis for multilevel phase-change memories in a single material system.
wo-dimensional (2D) layered materials have received extensive interest over the past few years. Particularly, besides graphene, many layered materials, such as MoS2, Bi2Se3, and TiSe2, are being studied with expectations of novel properties arising from reduced material dimensions.1−3 These 2D thin layers are usually obtained by “Scotch-tape” mechanical exfoliations, in view of their layered structures with the interlayer bonding characterized by weak van der Waals interactions. Previous studies have focused on their optical, electronic, and thermal properties; however, structural properties, especially the phase transformations in the context of phase-change memory applications, have received less attention. The 2D geometry can lead to unusual phase transformation characteristics and provides an ideal platform for studying the scaling of phase-change memories down to a few nanometers. Distinct from various phase-change materials (e.g., Ge−Sb− Te alloys) that are being explored for random-access memory applications, In2Se3 is polymorphic and has several crystalline phases (e.g., α, β, and γ phases) at various temperatures.4−8 Although In2Se3 phase-change memories have been demonstrated,9,10 the multilevel switching has not been extensively explored. If some of these crystalline phases can persist at room temperature (RT) and their electrical properties are significantly different from each other, it is possible to construct multilevel memories based on a single material system.11 In In2Se3, the α phase, which has a layered structure (schematic structure shown in Figure 1a), is stable at RT. The α phase has been shown to transform into the β phase at 473 K, which has © 2013 American Chemical Society
Received: March 8, 2013 Revised: June 30, 2013 Published: July 10, 2013 3501
dx.doi.org/10.1021/nl400888p | Nano Lett. 2013, 13, 3501−3505
Nano Letters
Letter
Figure 1. (a) Schematic In2Se3 layered crystal structure; (b and c) AFM images of exfoliated In2Se3 layers and corresponding line profiles taken along the dashed lines in the images.
Table 1. Lattice Parameters of α and β Phases literature (ref 12) α a (Å) Δa/a c (Å) Δc/c
β
4.025
4
this work α
β
4.205
4.166
−0.6% 28.762
−0.9% 28.33
−1.5%
and larger, where reasonable Raman signal-to-noise ratios can be achieved without damaging the layers. The structure of the layers was studied by correlated transmission electron microscopy (TEM) and micro-Raman spectroscopy. Particularly, the layers were exfoliated onto 50nm-thick SiNx membranes to allow for both TEM and Raman measurements on the same layers. The electron diffraction patterns of as-fabricated layers, shown in Figure 2a and b, are consistent with the single-crystal nature of the layer. The hexagonal pattern suggests the crystal orientation, that is, the normal to the layer plane, to be [001]. This is expected from the structure shown in Figure 1a. We note that, when looking along the [001] zone axis, diffraction spots including (100), (1−10), (0−10), (−100), (−110), and (010) planes should be observed, but they are very weak in our layers. The dominant spots observed (and labeled) in Figure 2a correspond to (110), (2−10), (1−20), (−1−10), (−210), and (−120) planes. The diffraction pattern of a different as-fabricated layer (Figure 2b) shows weak (but discernible) Bragg spots of the (100) planes. From the pattern indexing, the a lattice constant was extracted (Table 1) and is close to that obtained from bulk polycrystalline α-phase In2Se3.
28.742
28.213 −1.8%
The schematic crystal structure of α-phase In2Se3 is shown in Figure 1a, which belongs to the R3m space group.12 Each quintuple (Se−In−Se−In−Se) layer is bonded to each other through weak van der Waals interactions. Using the “Scotchtape” approach, thin In2Se3 layers were exfoliated from α-phase In2Se3 powders (Alfa Aesar, 99.99%) onto Si3N4/Si and SiO2/ Si substrates. Atomic force microscopy (AFM) images of representative thin layers are shown in Figure 1b and c. Although it is possible to obtain a single quintuple layer, we have found that ultrathin layers (thickness
