Ultrathin Topological Insulator Bi₂Se₃ Nanoribbons Exfoliated by Atomic Force Microscopy

Abstract

Ultra-thin topological insulator nanostructures, in which coupling between top and bottom surface states takes place, are of great intellectual and practical importance. Due to the weak Van der Waals interaction between adjacent quintuple layers (QLs), the layered bismuth selenide (Bi 2 Se 3 ), a single Dirac-cone topological insulator with a large bulk gap, can be exfoliated down to a few QLs. In this paper, we report the first controlled mechanical exfoliation of Bi 2 Se 3 nanoribbons (> 50 QLs) by an atomic force microscope (AFM) tip down to a single QL. Microwave impedance microscopy is employed to map out the local conductivity of such ultra-thin nanoribbons, showing drastic difference in sheet resistance between 1~2 QLs and 4~5 QLs. Transport measurement carried out on an exfoliated (≤5 QLs) Bi 2 Se 3 device shows nonmetallic temperature dependence of resistance, in sharp contrast to the metallic behavior seen in thick (>50 QLs) ribbons. These AFM-exfoliated thin nanoribbons afford interesting candidates for studying the transition from quantum spin Hall surface to edge states.Keywords: Topological insulator, Bismuth selenide, Nanoribbon, Mechanical exfoliation, Atomic force microscopy The metallic surface states of 3D topological insulators 1-7 are protected from disorder effects such as crystal defects and non-magnetic impurities, promising realization of dissipationless electron transport in the absence of high magnetic fields 8 . After the initial discovery of the 2D quantum spin Hall effect (QSHE) in HgTe quantum wells 7,9 , three binary compounds -Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 were predicted and later confi rmed by angle-resolved photoemission spectroscopy (ARPES) as 3D topological insulators 10-13 . In particular, Bi 2 Se 3 has been studied due to the relatively large bulk band gap (~0.3 eV) and the simple band structure near the Dirac point. 12-17 Many exotic physical phenomena are predicted to emerge in low dimensional nanostructures of Bi 2 Se 3 . 18,19 For example, ultra-thin Bi 2 Se 3 down to a few (≤5) nanometers is expected to exhibit topologically non-trivial edge states, which serves as a new platform for the 2D QSHE 18 .In addition, tuning of the chemical potential becomes easier than thick Bi 2 Se 3 due to the suppression of bulk contribution. Fortunately, such ultra-thin Bi 2 Se 3 can be naturally obtained due to its layered rhombohedral crystal structure; two Bi and three Se atomic sheets are covalently bonded to form one quintuple layer (QL, ~1 nm thick) (Figure 1b), where adjacent QLs are coupled by relatively weak van der Waals interaction. Such anisotropic bonding structure implies that similar to the case of graphene, 20 low dimensional crystals of Bi 2 Se 3 can be generated by mechanical exfoliation, which has been achieved by several groups [21][22][23] . However, the obtained flakes are usually irregular in shape and the yield of obtaining ultra-thin flakes is low: the typical reported flakes (~10 nm) are still thick compared with the 2D limit where strong coupling betwe...

Ferromagnetic two-dimensional crystals: Single layers of K2CuF4

“…Compared to other 2D crystals, the bilayer binding is about two to three times larger than in graphite [22,23] and 1.5 -2 times larger than in Bi 2 Se 3 [24], as obtained from theory. Both materials [25,26] and many other [1,27] allow for the isolation of 2D crystals by mechanical exfoliation techniques, and we infer from our calculations that the interlayer binding in K 2 CuF 4 is weak enough to exfoliate single layers of K 2 CuF 4 as well. Similar as for other 2D crystals, a possible way to produce single layers of K 2 CuF 4 could be the Scotch tape method.…”

Topological insulators and superconductors

“…Besides theoretical interest, this problem is also relevant to experiments, especially in the Bi 2 Se 3 family of materials. Indeed, these materials are layered and can be easily grown as thin films either by MBE (Li et al, 2010(Li et al, , 2009, catalyst-free vapor-solid growth , or by mechanical exfoliation (Hong et al, 2010;Shahil et al, 2010;Teweldebrhan et al, 2010). Several theoretical works studied thin films of the Bi 2 Se 3 family of topological insulators (Linder et al, 2009;Lu et al, 2010).…”

“…In addition to bulk samples, Bi 2 Se 3 nanoribbons (Hong et al, 2010;Peng et al, 2010) have been fabricated in the Cui group at Stanford University, and thin films of Bi 2 Se 3 and Bi 2 Te 3 have been grown by MBE by the Xue group at Tsinghua University (Li et al, 2009;, as well as other groups (Li et al, 2010;. Thin films can also be obtained by exfoliation from bulk samples (Hong et al, 2010;Shahil et al, 2010;Teweldebrhan et al, 2010). The stoichiometric compounds Bi 2 Se 3 , Bi 2 Te 3 , Sb 2 Te 3 are not extremely difficult to grow, which should allow more experimental groups to have access to high-quality topological insulator samples .…”

Influence of Chemical Composition of Liquid Phase and Growth Process on Physical Properties of Bi2Se3, Bi2Te3and Bi2Te2Se Compounds

“…One quintuple thickness is about 1 nm. It is according to data presented by Kim et al [7] and Hong et al [8].…”