Article pubs.acs.org/cm
Tunable Bandgap Narrowing Induced by Controlled Molecular Thickness in 2D Mica Nanosheets Sang Sub Kim,*,† Tran Van Khai,‡ Vadym Kulish,§ Yoon-Hyun Kim,∥,⊥ Han Gil Na,‡ Akash Katoch,† Minoru Osada,*,∥,⊥ Ping Wu,*,§ and Hyoun Woo Kim*,‡ †
Department of Materials Science and Engineering, Inha University, Incheon 402-751, Republic of Korea Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Republic of Korea § Entropic Interface Group, Singapore University of Technology & Design, Singapore 138682, Singapore ∥ International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan ⊥ Graduate School of Advanced Science and Engineering, Waseda University, Shinjyu-ku, Tokyo 169−8555, Japan ‡
S Supporting Information *
ABSTRACT: Bandgap engineering of atomically thin 2D crystals is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here, we report a simple but rather unexpected approach for bandgap engineering of muscovite-type mica nanosheets (KAl3Si3O10(OH)2) via controlled molecular thickness. Through density functional calculations, we analyze electronic structures in 2D mica nanosheets and develop a general picture for tunable bandgap narrowing induced by controlled molecular thickness. From conducting atomic force microscopy, we observe an abnormal bandgap narrowing in 2D mica nanosheets, contrary to wellknown quantum size effects. In mica nanosheets, decreasing the number of layers results in reduced bandgap energy from 7 to 2.5 eV, and the bilayer case exhibits a semiconducting nature with ∼2.5 eV. Structural modeling by transmission electron microscopy and density functional calculations reveal that this bandgap narrowing can be defined as a consequence of lattice relaxations as well as surface doping effects. These bandgap engineered 2D mica nanosheets open up an exciting opportunity for new physical properties in 2D materials and may find diverse applications in 2D electronic/optoelectronic devices.
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INTRODUCTION Two-dimensional (2D) nanosheets with atomic or molecular thickness are emerging as important new materials because of their particular properties and potential applications in nextgeneration electronic devices.1−12 One attractive aspect of these exfoliated nanosheets is that various nanostructures can be fabricated using them as 2D building blocks. Sophisticated functionalities or nanodevices may be designed through combining different nanosheets with a precise control over their arrangement on a molecular scale. The discovery of graphene can be considered a defining point in the research and development of such 2D material systems.1,2,12 This breakthrough has opened up the possibility of exploring the fascinating properties of 2D nanosheets of other inorganic layered materials;2−11 the reduction to single or a few atomic layers will offer new properties and novel applications.13 To expand the utility of these 2D nanosheets, the electronic properties must be tailored through bandgap engineering and/ or doping process. Bandgap engineering of 2D nanosheets is particularly important for their applications in nanoelectronics, optoelectronics, and photonics. One key issue in the developments of 2D nanosheets is to produce semiconductor © XXXX American Chemical Society
nanosheets with a narrow bandgap or a semiconductor-tometal transition, since it allows the use of field effect transistors (FETs) as well as the effective operation for low-energy absorptions and excitation of semiconductor optoelectronics. A possible indication of the bandgap engineering came from MoS2 nanosheets, which exhibited a crossover behavior from an indirect to a direct-gap semiconductor in the monolayer limit.14 However, the bandgap narrowing of nanomaterials is almost always difficult to achieve, since most nanomaterials would show an enlarged bandgap due to the quantum confinement effects.15−21 Here, we report a simple but rather unexpected approach for tunable bandgap narrowing in 2D mica nanosheets using controlled molecular thickness. Mica, a mineral, has been used in various industrial applications including insulating substrates, capacitors, paint films, and barrier coating. There are several kinds of mica with different properties, but mica is in general very stable electrically, mechanically, and chemically. Mica is Received: December 31, 2014 Revised: May 22, 2015
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DOI: 10.1021/cm504802j Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
Figure 1. Crystal structures of muscovite-type mica (KAl3Si3O10(OH)2) and its exfoliated nanosheets with controlled molecular thickness (n).
eV.23,24 Due to the well-known shortcomings of the DFT calculation in the generalized gradient approximation (GGA), our electronic structure underestimates the bandgap energy. Similarly, previous literature indicated that the DFT-calculated bandgaps are much smaller than experimentally measured ones for various semiconductors, including ZnO and ZrO 2 (Supporting Information, Text S1). However, the comparison with the bulk case gives some idea to follow the thickness dependence of electronic properties in mica nanosheets. In the nanosheet cases with different layers (n = 1, 2, 3), our calculations clearly indicated that the bandgap energy is highly sensitive to the thickness (Figure 2b−d). For n = 1, 2, and 3, general band features were similar to the bulk case; the valence band (VB) is mainly composed of Op orbitals, while the other elements (K, Al, Si) contribute to the conduction band (CB). In the nanosheets, however, the bandgap feature was smeared out; the bandgap energy was reduced with molecular thickness (n). The monolayer nanosheet exhibited a semimetallic band structure with a small bandgap of 0.5 eV (Figure 2b). In comparison with the bulk case, the calculated density of states (DOS) for the monolayer clearly indicated that the K ions give broad bands near the CB edge, at the expense of reduced Al and Si bands. In a few layers of mica nanosheets (n = 2, 3), the bandgap energy was linearly increased with molecular thickness (n); the bandgap energies for n = 2 and n = 3 were 0.81 and 1.02 eV, respectively. We also observed significant K states at the CB minimum, whereas the Si and Al states gave a large density near the CB edge, a situation almost identical to the bulk case (Figure 2c,d). These broad pre-edge structures of the K states are a particular feature from the surface contribution of nanosheets and attributed to the strongly hybridized donor impurity band, causing a bandgap narrowing. Although it would be useful to calculate DOS for thicker mica layers, unfortunately it cannot be done due to limitations of computational
one of the oldest dielectric materials, which has a wide bandgap of 7.85 eV with a dielectric constant of 6−9.22−24 Because of its unique structures and properties, it has been tempting to try to split or cleave the bulk mica into very thin sheets or nanosheets.25,26 Such ultrathin mica sheets have been applied for mica capacitors. Although the delamination of mica has a long history, it is still unclear if free-standing single or few layer nanosheets could exist? If this is the case, how does the reduced dimension and thickness affect physical properties? These issues are of particular interest for both basic and practical viewpoints since they might shed new light on new properties and novel applications of this old material. In this study, we present comprehensive investigations on structural and electronic properties in muscovite-type mica nanosheets (KAl3Si3O10(OH)2) with controlled molecular thickness (Figure 1). Through first-principles density functional theory (DFT) calculations, we analyze electronic structures in 2D mica nanosheets and develop a general picture for tunable bandgap narrowing induced by controlled molecular thickness. We predict and explain a crucial role of both lattice relaxations and surface doping effects in the observed bandgap narrowing in 2D mica nanosheets. Finally, we confirm our predictions experimentally.
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RESULTS AND DISCUSSION First-Principles Calculation and Electronic Structures of 2D Mica Nanosheets. We carried out first-principles DFT calculations on structural and electronic properties in 2D mica nanosheets with controlled molecular thickness (Figure 2). In the bulk case (Figure 2a), the band structure was composed of the empty d or s orbitals of metal atoms (K, Al, Si) and the filled p orbitals of oxygen. The calculated bandgap was about 3.2 eV, being much smaller than the experimental value of 7.85 B
DOI: 10.1021/cm504802j Chem. Mater. XXXX, XXX, XXX−XXX
Article
Chemistry of Materials
mica. The lateral size was approximately 100−500 nm. Structural characterizations by X-ray diffraction (XRD), highresolution transmission electron microscopy (HRTEM), and selected area electron diffraction (SAED) revealed that the exfoliated products are highly crystalline nanosheets with various thicknesses (Figure 3b−e). In XRD and SAED, all of the observable peaks were indexable to the (00l) reflections of a monoclinic cell. In HRTEM, three lattice spaces of 0.45 nm were observed, which correspond to the (11), (11), and (20) planes of mica. These results indicate that the structure of the host layers was preserved during the exfoliation process. Chemical analyses by energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of the O, Al, Si, and K elements (Figures S1, S2), which are the main constituents of muscovitetype mica. We also note the change in chemical composition with the thickness; the reduced K concentration with decreasing the thickness (Tables S1, S2). In the nanosheets, the half portion of K ions still remains at the exposed surface. The exfoliation process brings the exposed surface into contact with the ambient environment, releasing K+ ions. The thinner nanosheets possess a higher surface-to-volume ratio, causing a reduced overall K concentration. A mechanical exfoliation technique was also utilized for synthesizing mica nanosheets (Figures S3, S4). From TEM and XRD, we confirmed that structural features are almost identical to those obtained by the chemical exfoliation. Structural characterizations by XRD, HRTEM, and SAED detected fewlayered nanosheets (n ≤ 5) with a monoclinic muscovite-type structure (Figures S5−S9). The Raman spectrum of mechanically exfoliated mica was similar to that of pristine mica (Figure S9b). These results indicate that the structure of the host layers was preserved during the mechanical exfoliation process. We note that these structural features of mica nanosheets are not dependent on exfoliation techniques, and mica nanosheets with controlled molecular thickness can be achieved by both chemical and mechanical exfoliation techniques. Bandgap Engineering Induced by Controlled Molecular Thickness. To investigate the bandgap of mica nanosheets, we performed conducting AFM (c-AFM). c-AFM allows the differentiation of the local electronic structure (Figure S10).29 This is particularly important for investigation of mica nanosheets, which possess different molecular thicknesses. Figure 4 presents (dI/dV)−V curves obtained for mica nanosheets with n = 2, 3, 5, and 10. Complementary I−V data are also shown in Figure S11. According to the Wentzel− Kramers Brillouin approximation, the structure in the I−V or (dI/dV)−V curve, as a function of the tip−sample bias (V), is associated with the density of states (DOS) of the sample when the tip−sample bias is less than the work function of the tip and the sample.30,31 Semiconductors show a highly bent DOS; the conductance around the Fermi level is zero, where the threshold voltage is associated with the band gap Eg = |V+bias| + |V−bias|. Therefore, the band gap of the sample can be determined from the location of the I−V measurement. Alternatively, a plot of (dI/dV)−V allows us to locate the band edges that determine the band gap. For the thicker cases (n ≥ 5), the nanosheet exhibited a high resistance with a current level of
