Synthesis and Optical Properties of MoS 2 Nanoclusters
- PDF / 411,906 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 106 Downloads / 197 Views
Mat. Res. Soc. Symp. Proc. Vol. 452 @1997 Materials Research Society
calibrated columns (for the smallest clusters). As grown cluster samples were purified using high pressure liquid chromatography. RESULTS AND DISCUSSION Optical Absorption of MoS, Figure 1 shows the absorption spectra of different MoS 2 cluster samples and compares these spectra to those of two bulk crystalline samples taken from Refs. 7 and 8. The spectrum of bulk MoS 2 consists of a series of absorption thresholds the first of which corresponds to weak absorption in the near IR at -1040 nm (-1.2 eV) associated with an indirect gap between F and the middle of the Brillouin zone between F and K. The second threshold occurs at -700 nm (-1.8 eV) and is associated with a direct transition at the K point.3 The two peaks (A1 and B,) on the short wavelength (X) side of this threshold are the first (n=l) members of two excitonic Rydberg series corresponding to the transitions K4-*K5 and Kl-*K5, respectively, whose energy separation (-0.20 eV) is due to the spin-orbit splitting of the top of the valence band at K.3'7 A third threshold at -500 nm (-2.5 eV) is due to a direct transition from deep in the valence band to the conduction band. Excitonic features (C and D) are also associated with this transition. A fourth threshold at -350 nm (-3.5 eV) is also due to transitions from deep in the valence band. Features on the high energy side of this threshold (e.g., E in Figure 1) are seen in thin samples as illustrated in curves 2' and 2".` The cluster results in Fig. 1 show that the main absorption features in this spectral region remain essentially unaltered, but exhibit large blue shifts with decreasing cluster size. Because our measurements were made on dilute solutions (1V to 10-3) molar, and because of the inherent weak absorption characteristic of indirect transitions, the signal-to-noise ratio for the cluster samples did not allow measurements in the region of the indirect gap. Nevertheless, the shape of the cluster spectra on the short X side of this gap suggests that the gap remains indirect. Figure 1 shows the evolution of the room temperature absorption spectrum with decreasing size reveals a one-to-one correlation between the bulk and cluster spectra.4 The resolved features in the spectra of the purified samples in this figure make it clear that the bulk-like excitonic features are retained to smaller cluster sizes than originally thought.6 The earlier spectra of unpurified samples suggested that a bulk-like to molecule-like crossover occurs in the 3 to 2.5 nm size range. The present results on purified samples suggest that this expected crossover occurs below 2.5 nm.4 This is a remarkable result which indicates that clusters which are only two unit cells thick have enough densities of states to reproduce bulk-like absorption properties, a feature undoubtedly related to the 2-D nature of MoS 2. A variety of theoretical models have been used to describe size quantization in semiconductor clusters.' They include effective mass (EM) treatments, sem
Data Loading...
