Atomic- and electronic-structure study on the layers of 4 Hb -TaS 2 prepared by a layer-by-layer etching technique Ju-Jin Kim and H. Olin Department of Physics, Chalmers University of Technology, and Gothenburg University, S-41296, Gothenburg, Sweden Received 19 July 1995 We have studied the atomic and electronic structures of 4 Hb -TaS 2 , which has alternating layers of the 1 T and 1 H type, at room temperature and 77 K, using a scanning tunneling microscope. Using a layer-by-layer etching technique, we fabricated staircases with alternating layers of the 1 T and 1 H type. The T -type layers showed the typical  13 13 charge-density-wave structures, whereas the H-type layers had the triangular atomic structure at both temperatures. The measured tunneling spectra of each layer at 77 K showed entirely different characteristics; the 1 H layer remained in the metallic state, whereas the 1 T layer showed an insulat- ing behavior with a wide opening of the energy gap at the Fermi level at 77 K. Among the transition-metal dichalcogenides, the poly- type 4 Hb -TaS 2 and 4 Hb -TaSe 2 show interesting atomic and electronic properties, which are believed to mainly originate from the alternating layers of the trigonal prismatic coordi- nation (1 H ) and octahedral coordination (1 T ). 1,2 These properties reflect the composite nature of those observed in the pure octahedral phases such as 1 T -TaS 2 ,1 T -TaSe 2 and the trigonal prismatic phases such as the 2 H -TaS 2 ,2 H - TaSe 2 . Theoretical calculations and experimental results 2–4 show that each layer maintains its characteristic features found in the corresponding pure phases, although there has been a small electron transfer between the two different lay- ers. Particularly, the scanning tunneling microscope STM studies by Giambattista et al. 5 and Coleman et al. 2 showed two completely different kinds of images, one kind with a  13 13 charge-density wave CDW and another kind with atomic structures, on the two opposite faces of the same crystal cleave, presumably representing the 1 T -type and 1 H - type layers. They also showed a mixing of the two images and sometimes a time-dependent change of the image, which might be due to the complex superposition of two images. 2,6 Recently, Han et al. 7 observed strong bias-dependent STM images at room temperature. At relatively high positive bias voltage, a fully developed CDW modulation was observed even on the presumably 1 H surface, which was explained in terms of an energy-dependent tunneling process between 1 T and 1 H layers. Therefore, it is very important to identify each layer precisely, that is, whether the top layer is 1 T or 1 H . Tanaka and co-workers 8,9 tried to distinguish 1 T from 1 H layers using tunneling spectroscopy and obtained char- acteristic spectra corresponding to the 1 H - and 1 T -type lay- ers at room temperature. However, the observed tunneling spectra at room temperature are rather obscure due to large thermal fluctuations and an interlayer tunneling effect be- tween the neighboring layers, making it difficult to distin- guish each layer precisely. Layer-by-layer etching of the surface of transition-metal dichalcogenides has been used earlier using STM Refs. 10 and 11 and atomic force microscope. 12 Due to the weak van der Waals force between each layer, it is possible to etch away individual layers in a well-defined manner. Using this technique, it should be possible to choose each layer in the poly-type transition-metal dichalcogenides by making suc- cessive layer-by-layer removal. In this study, we have etched a staircase of several layers of 4 Hb -TaS 2 by a layer-by-layer etching technique and in- vestigated the electronic and atomic structures on the newly etched region at room temperature and 77 K. The method allows us to clearly distinguish between the two types of layers that exist. The measured STM images and tunneling spectra showed the alternating nature of the 1 T - and 1 H - type layers. The 1 T -type layers showed the typical  13 13 CDW structures, whereas the 1 H -type layers had triangular atomic structure with a weakly superposed CDW superlattice at relatively high bias voltage. The measured tunneling spectra on each layer at 77 K showed entirely dif- ferent characteristics between the two layers; the 1H layer remained in the metallic state, whereas the 1 T layer showed an insulating behavior with a wide opening of an energy gap at the Fermi level at 77 K. Single crystals of 4 Hb -TaS 2 were grown by the usual iodine transport method. The samples were cleaved at room temperature in air and set in the STM unit. For the room- temperature experiments, it was placed in the center of a doubly shielded cryostat in vacuum or in a He exchange gas environment. The STM unit was cooled very slowly to 77 K in the high vacuum state for low-temperature experiments. Mechanically polished Pt/Ir tips were used. All images were obtained in the constant current mode. Figure 1a shows an STM image 1250 Å1250 Å of the 4 Hb -TaS 2 single crystal. The tunneling current and bias voltage were 10 nA and 32 mV, respectively. The etching was performed during scanning. First, the full area was scanned while the whole etching processes were monitored by STM imaging. Small defect regions grew very slowly as the tip was scanned across the layer and suddenly, a large part of the layer was removed. After removing, we measured STM images on this specific layer but with a smaller scan area. After then, we returned to etching of the next layer with the full area scan, and so on. Continuing the scanning and measuring process about 40-min scan resulted in the re- PHYSICAL REVIEW B 15 NOVEMBER 1995-II VOLUME 52, NUMBER 20 52 0163-1829/95/5220/143884/$06.00 R14 388 © 1995 The American Physical Society