Observation of a low temperature n–p transition in individual titania nanotubes

Abstract

Manipulating the transport properties of titania nanotubes (NTs) is paramount in guaranteeing the material's successful implementation in various solid state applications. Here we present the unique semiconducting properties of individual titania NTs as revealed from thermoelectric and structural studies performed on the same individual NTs. The NTs were in the anatase phase fabricated by anodic oxidation and doped with intrinsic defects created by reducing the lattice thermally. Despite their polycrystalline nature and nanoscale walls, the doped NTs were found to be 4-5 orders of magnitude more electrically conducting than TiO nanowires and thin films, with values approaching the bulk single crystal conductivity. The reason for the high conductivity was found to be the high carrier concentration on the order of 10 cm, which counteracted the low mobility values ∼0.006 cm V s. Furthermore, this high level of carrier concentration transitioned the NTs to a degenerate state, which is the first such example in thermally doped titania NTs. More importantly, our study showed the creation of acceptor states along with donor states in individual nanotubes upon lattice reduction. These acceptor levels were found to be active at low temperatures when donor states were not ionized, shifting the Fermi level (E) from the conduction band to the valence band.

Step-by-Step Improving Photocatalytic Hydrogen Evolution Activity of NH2–UiO-66 by Constructing Heterojunction and Encapsulating Carbon Nanodots

“…The electronic band structures of g-C 3 N 4 , NH 2 –UiO-66/g-C 3 N 4 , and CD@NH 2 –UiO-66/g-C 3 N 4 were further investigated by examining Mott–Schottky plots. As shown in Figure c, CD@NH 2 –UiO-66/g-C 3 N 4 , NH 2 –UiO-66/g-C 3 N 4 , NH 2 –UiO-66, and g-C 3 N 4 exhibited positive slopes in the Mott–Schottky plots at frequencies of 500 Hz, which was an indication of the n-type semiconductors . The results of Mott–Schottky measurements indicate that the flat band position of NH 2 –UiO-66, g-C 3 N 4 , NH 2 –UiO-66/g-C 3 N 4 , and CD@NH 2 –UiO-66/g-C 3 N 4 are about −0.79, −1.15, −0.72, and −0.68 V, respectively, with reference to the saturated calomel electrode (SCE).…”

“…As shown in Figure 2c, CD@NH 2 −UiO-66/g-C 3 N 4 , NH 2 −UiO-66/g-C 3 N 4 , NH 2 −UiO-66, and g-C 3 N 4 exhibited positive slopes in the Mott−Schottky plots at frequencies of 500 Hz, which was an indication of the n-type semiconductors. 49 The results of Mott−Schottky measurements indicate that the flat band position of NH 2 −UiO-66, g-C 3 N 4 , NH 2 −UiO-66/g-C 3 N 4 , and CD@NH 2 −UiO-66/g-C 3 N 4 are about −0.79, −1.15, −0.72, and −0.68 V, respectively, with reference to the saturated calomel electrode (SCE). It is widely recognized that the bottom of the conduction band in many n-type semiconductors is more negative by 0.15 V than the flat band potential.…”

Bipolar Thermoelectrical Transport of SnSe Nanoplate in Low Temperature*

“…The bipolar transport indicates that both electrons and holes contribute to electrical and thermoelectrical properties at the same time. [35,36] The measured Seebeck coefficient is a combination of the negative Seebeck coefficient from the electrons and the positive Seebeck coefficient from the holes. [37,38] The sign change of the total Seebeck coefficient is determined by the proportion of the electron Seebeck coefficient and the hole Seebeck coefficient.…”

Using Mosaicity to Tune Thermal Transport in Polycrystalline Aluminum Nitride Thin Films

“…31,32 The reduced thermal conductivity of AlN thin films, is due to increased phonon scattering rate at the grain boundaries among others, impurity scattering, and micro-structural defects. To quantify the true contribution of the various phonon scattering parameters we used the Callaway/Klemens' formulation 5,33 derived from Boltzmann transport equation used previously 34,35 which was modified and discussed in detail below.…”