Charles M. Lieber

Characterization of nanometer scale wear and oxidation of transition metal dichalcogenide lubricants by atomic force microscopy

Atomic force microscopy has been used to characterize wear and oxidation of transition metal dichalcogenide surfaces. Sequential images recorded on molybdenum disulfide (MoS2) and niobium diselenide (NbSe2) surfaces show that wear proceeds at defects, and that MoS2 wears at least five times more slowly than NbSe2. Images of thermally treated MoS2 and NbSe2 further demonstrate that oxidation creates surface defects on both materials. However, for similar oxidation conditions, NbSe2 surfaces show extensive degradation, while MoS2 surfaces only exhibit isolated defects. The implications of these results to understanding the tribological properties of the transition metal dichalcogenides are discussed.

(RbxK1–x)3C60 Superconductors: Formation of a Continuous Series of Solid Solutions

By means of an approach that employs alkali-metal alloys, bulk single-phase (RbxK1–x)3C6O superconductors have been prepared for all x between 0 and 1. For x = 1 it is shown that the maximum superconducting fraction, which approaches 100% in sintered pellets, occurs at a Rb to C60 ratio of 3:1. More importantly, single-phase superconductors are formed at all intermediate values of x, and it is shown that the transition temperature (Tc) increases linearly with x in this series of materials. The formation of a continuous range of solid solutions demonstrates that the rubidium- and potassium-doped C60 superconducting phases must be isostructural, and furthermore, suggests that the linear increase in Tc with x results from a chemical pressure effect.

Tunneling Spectroscopy of M3C60 Superconductors: The Energy Gap, Strong Coupling, and Superconductivity.

Tunneling spectroscopy has been used to characterize the magnitude and temperature dependence of the superconducting energy gap (▵) for K3C60 and Rb3C60. At low temperature the reduced energy gap, 2▵κTc (where Tc is the transition temperature) has a value of 5.3 � 0.2 and 5.2 � 0.3 for K3C60 and Rb3C60, respectively. The magnitude of the reduced gap for these materials is significantly larger than the value of 3.53 predicted by Bardeen-Cooper-Schrieffer theory. Hence, these results show that the pair-coupling interaction is strong in the M3C60 superconductors. In addition, measurements of ▵(T) for both K3C60 and Rb3C60 exhibit a similar mean-field temperature dependence. The characterization of ▵ and ▵(T) for K3C60 and Rb3C60 provides essential constraints for theories evolving to describe superconductivity in the M3C60 materials.

Structural and Electronic Role of Lead in (PbBi)2Sr2CaCu2O8 Superconductors by STM.

The structural and electronic effects of lead substitution in the high-temperature superconducting materials PbxBi2-xSr2CaCu2O8 have been characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Large-area STM images of the Bi(Pb)—O layers show that lead substitution distorts and disorders the one-dimensional superlattice found in these materials. Atomic-resolution images indicate that extra oxygen atoms are present in the Bi(Pb)—O layers. STS data show that the electronic structure of the Bi(Pb)—O layers is insensitive to lead substitution within �0.5 electron volt of the Fermi level; however, a systematic decrease in the density of states is observed at ≈1 electron volt above the Fermi level. Because the superconducting transition temperatures are independent of x(Pb) (x ≤ 0.7), these microscopic STM and STS data suggest that the lead-induced electronic and structural changes in the Bi(Pb)—O layer do not perturb the electronic states critical to forming the superconducting state in this system.

Characterization of the structural, electronic and tribological properties of metal dichalcogenides by scanning probe microscopies

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have been used to characterize the structural and electronic properties of single-crystal MoS2, nickel-substituted MoS2 (NixMo1−xS2), and chalcogenide-substituted MoS2 (MoS2−xChx, Ch ≡ Se, Te) at the atomic level. Images of Ni0.1Mo0.9S2 demonstrate that nickel substitution causes localized changes in the electronic states, although the structure of the surface sulfur layer is unchanged compared with MoS2. Investigations of MoS1.75Se0.25 also show that within the detection limits of AFM selenium substitution does not perturb the sulfur surface structure, and STM data further indicate that the substituted selenium is electronically delocalized. In contrast, AFM studies of MoS1.75Te0.25 show that tellurium substitution produces atomic-sized structural protrusions that may modify significantly the tribological properties of MoS2. In addition, we demonstrate that material wear can be characterized on an atomic scale by AFM. These studies indicate that the microscopic origin of material wear as well as the local structure and electronic properties should be considered to develop further models of friction and wear in metal dichalcogenide materials.

Variable temperature scanning tunneling microscopy studies of the charge density wave phases in tantalum disulfide

Variable temperature scanning tunneling microscopy has been used to elucidate details of the nearly commensurate charge density wave (CDW) phase in 1T‐TaS2. Large‐area images show that the nearly commensurate phase has a hexagonal domain structure, and that the domain period exhibits a strong temperature dependence. Real‐space and two‐dimensional Fourier transform analyses of atomic resolution images further demonstrate that within the domains the CDW is approximately commensurate with the atomic lattice, and that between domains the CDW amplitude decreases and the CDW phase changes. In addition, disorder observed in the hexagonal domain structure at low temperatures is suggested to be due to domain wall pinning.

Scanning tunneling microscopy investigations of the surface structure and electronic properties of ternary graphite intercalation compounds

Scanning tunneling microscopy has been used to characterize the surface structure of the KHgC4 (stage 1) and KHgC8 (stage 2) graphite intercalation compounds. Images of the stage 1 and stage 2 materials exhibit a new commensurate 2×2 superlattice in addition to the centered hexagonal lattice observed in images of pristine graphite. Consideration of the intercalant layer structure and previous studies of stage 1 (MC8) and stage 2 (MC24) alkali metal graphite intercalation compounds (GICs) indicate that the 2×2 superlattice is due to a modulation of the surface carbon layer density of states by the periodic (2×2) potential of the potassium ions. In addition, a new orthorhombic superlattice, a=b=0.89 nm and <a−b≂89°, has been observed in images of the stage 1 KHgC4 material. Possible origins of this novel superstructure are discussed.

Scanning tunneling microscopy and spectroscopy studies of the surface structure and electronic properties of single‐crystal Tl–Ba–Ca–Cu–O superconductors

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) have been used to characterize the surface structure and electronic properties of Tl2Ba2CaCu2O8 and Tl2Ba2Ca2Cu3O10 single crystals. Atomic resolution STM images show that 90%–95% of the surface has a near‐trigonal structure with a=b=0.24 nm and an a–b angle of 65°. The remaining 5%–10% of the surface consists of a new orthorhombic structure with a=b=0.40 nm. Spatially resolved STS measurements further demonstrate that regions with the 0.24 nm period structure are metallic, while areas with the 0.40 nm period structure are semiconducting.

THE ENERGY GAP OF THE M3C60 SUPERCONDUCTORS

We briefly review tunneling spectroscopy studies of the new fullerene-based superconductors, K3C60 and Rb30C60 from our laboratory. At low temperature the reduced energy gap, 2∆/kTc, has a value of 5.3 ± 0.2 and 5.2 ± 0.3 for K3C60 and Rb3C60, respectively. The magnitude of the reduced gap for these materials is significantly larger than the value of 3.53 predicted by BCS theory. These results demonstrate that the energy gap scales with Tc and show that the pair coupling interaction is strong in the M3C60 superconductors, and thus they provide information essential to understanding superconductivity in the M3C60 materials.