Electrical Conduction of Superatom Thin Films Composed of Group-V-Metal-Encapsulating Silicon-Cage Nanoclusters

Abstract

Nanocluster assembled films have attracted great interest for designing nanostructured materials with unique electromagnetic properties through a bottom-up approach. Superatoms of group-5 metals (M V = V, Nb, and Ta) encapsulating silicon (Si) cage nanoclusters (M V @Si 16 ), which are synthesized by high-power impulse magnetron sputtering technique, can be efficiently generated to form assembled films. Temperature-dependent current−voltage (I−V) characteristics of the M V @Si 16 assembled films revealed that the electrical conduction mechanism is not band transport but hopping transport with Efros−Shklovskii variable range hopping for all central M V atoms. The results show that electrons involved in conduction are strongly correlated to localized electronic states; this correlation arises because of not only the geometrical disordering in noncrystalline assembled films but also the electronic nature of a superatomic 1H orbital with multiple nodes. The localization length depends on the specific M V and is several times the radius of M V @Si 16 (0.45 nm); it is the largest for Ta (2.2 nm) and the smallest for Nb (0.8 nm), revealing a periodicity of superatoms.

Bridging the gas and condensed phases for metal-atom encapsulating silicon- and germanium-cage superatoms: electrical properties of assembled superatoms

“…Experimental methods are briefly described, as previously reported elsewhere. 33 With the use of HiPIMS technique described in Section 2, M@Si 16 cations or anions can be generated for more than 1 nA for various central metal as shown in the mass spectra of Figure S1. Owing to the alkali metal-like nature of M@Si 16 BCSA for group-5 metal-atom encapsulation, the cations of M@Si 16 + are preferably generated for M = V, Nb, and Ta, satisfying the 68 electron shell closure.…”

“…The electrical characteristics of group-5 metal-atom (M V = V, Nb and Ta) encapsulating Si cage BCSA (M V @Si 16 ) assembled films 33 conduction showed that ln G is not proportional to T −1 , indicating that M V @Si 16 films hardly form band structure. The electronic structures of isolated M V @Si 16 showed that singly occupied molecular orbital (SOMO) is superatomic 1H orbital with multiple nodes, making the electronic overlap of neighboring M V @Si 16 small.…”

“…In summary, we described the progress of NC research starting from the methodology of The assembled film fabrication by M@Si 16 BCSAs could be achieved with the high intensity NC source based on HiPIMS and soft landing technique. 33 High symmetric and electronic factor cooperatively stabilize M@Si 16 BCSAs, enabling their assembly without deforming a unique caged structure to characterize macroscopic characteristics such as electrical conduction. The central metal dependence of M@Si 16 was investigated along with germanium-cage SAs of M@Ge 16 assembled films in an amorphous state to shed some light on superatomic periodicity in assembled materials.…”

“…38 Furthermore, cage atoms can be altered from Si to Ge, enabling the tuning of cage properties. 39,40 Since our recent achievement in the electrical characterization of M@Si 16 assembled films with group-5 metal-atom encapsulation (M = V, Nb, and Ta), 33 we extended the characterization with different group metals and with Ge cage BCSAs to widen the perspective on M@Si 16 /M@Ge 16 SA-assembled films.…”

Size-Dependent Oxidative Stability of Silicon Nanoclusters Mixed with a Tantalum Atom

“…In particular, the electronic properties of M@Si 16 SAs have attracted much attention, as tuned by selecting the dopant central metal atoms. ,− M@Si 16 SAs can be selectively generated in the gas phase owing to their highly symmetric geometric structure and electron-shell closure of superatomic two-dimensional (2D) orbitals (68 electrons). , In fact, selective formation of M@Si 16 SAs in the gas phase has been demonstrated when the total count of valence electrons is 68 in either anionic, neutral, or cationic species; for example, Sc@Si 16 – , Ti@Si 16 , and V@Si 16 + . Recently, for use as functional nanomaterials, M@Si 16 SAs have been synthesized on the milligram scale and deposited on a substrate in a nondestructive manner, and these samples were chemically and electronically characterized in practical environments. − …”