Oxygen Vacancies Allow Tuning the Work Function of Vanadium Dioxide

Abstract

over a wide range by substitutional doping and external strain, etc. [3] Due to these remarkable features, VO 2 has in addition to its potential use as electrode, [4] applications such as optical switching devices, [5] memory devices, [6] and smart windows. [7] At room temperature (RT), undoped VO 2 is in the insulating state and for most VO 2 thin films the work function is moderately high (up to 5.55 eV). [8] Thus, VO 2 has not been considered as a suitable hole injecting electrode as for related hole injecting transition metal oxides, the work function is usually >6.0 eV. [9] However, recently we could show that the work function of VO 2 can reach up to 6.70 eV and, moreover, even in the insulating state at RT, the conductivity is sufficient to allow for adequate charge transport. [4] VO 2 is yet a complex material and although the MIT was discovered already in 1959, [2a] it is still under debate whether it is induced by Mott-Hubbard correlation or due to a structural Peierls-type transition, or both. [10] Consequently, also the underlying reasons that enable achieving high work function are largely unexplored. It can be speculated that for VO 2 , as for other transition metal oxides, the work function is related to the stoichiometry. [11] Vanadium, with an electronic configuration of [Ar]3d 3 4s 2 , can form multivalent oxides with different valence states, like VO, V 2 O 5 , V 2 O 3 , V 6 O 13 , and VO 2 . [12] It is thus essential to have a comprehensive understanding of the electronic properties of an actual surface in order to use it in practical devices. [13] The evolution of electron valence bands, core levels, and work function of vanadium dioxide (VO 2 ) thin films upon argon ion sputtering and annealing, as commonly done to obtain atomically clean surfaces, with and without low-pressure oxygen atmosphere during annealing is investigated by ultraviolet and X-ray photoemission spectroscopy. Both sputtering and annealing in vacuum introduce lower oxidation state V species, leading to an increased intensity of V 3d derived bands close to the Fermi level. Such oxygen deficient surfaces exhibit low work function values as low as 4.40 eV. Annealing the sample under low-pressure oxygen atmosphere (few times 10 −4 mbar partial O 2 pressure) results in stoichiometric VO 2 surfaces with a high work function of up to 6.70 eV. Moreover, the work function of the VO 2 can be continuously tuned between the high and low limits by adjusting the atomic ratio of oxygen and vanadium at the surface. Appropriately adjusted VO 2 can thus be employed as moderate electron as well as superior hole injecting electrode material in electronic and optoelectronic devices. Figure 6. Correlation of work function and atomic ratio (O to V). www.advancedsciencenews.com

Surface hydrogeneration of vanadium dioxide nanobeam to manipulate insulator-to-metal transition using hydrogen plasma

“…Figure 5(e) exhibits the calculated work function of VO 2 nanobeams for different hydrogen doping time. The work function of pristine VO 2 nanobeams is found to 5.39 eV, which is slightly higher than the reported value [23,43], and it decreases sequentially with increasing H-doping time. The lowest work function is found to be 5.29 eV for 10 min H doping time.…”

“…The lowest work function is found to be 5.29 eV for 10 min H doping time. From interpretation of the work function measured by KPFM from other recent studies, it has been found that the change in the work function is closely related to nearsurface stoichiometry (V/O ratio related to V valence) [23] or the density of point defect like oxygen vacancy in lattice [44]. In addition, the crystal structure is also considered a factor.…”

Atomically precise vanadium-oxide clusters

“…The most prominent method involves the removal of lattice oxygen atoms to form oxygen-vacant sites on the surface of the material. 14,15,[191][192][193] An alternative method to anionically-doping vanadium oxide materials is through the substitution of oxide moieties for non-oxygen anions such as halides, [194][195][196][197][198][199][200] sulfides, [201][202][203] or nitrides. 204,205 Experimental and computational investigations have shown that both physical and chemical properties are significantly influenced by the introduction of these anionic dopants.…”

Interplay among work function, electronic structure and stoichiometry in nanostructured VOx films

“…Vanadium ([Ar] 3d 3 4s 2 ) is a very reactive element and different oxides such as VO, V2O3, VO2 and V2O5 characterized by the oxidation states: V +2 , V +3 , V +4 and V +5 , respectively, can be synthetized. The WF of vanadium oxides can be tuned by changing the stoichiometry of the sample, 6,13,14 or its dimensionality. The nanoscale dimensions in thin films and nanoparticles can be an important degree of freedom to control the WF of any oxide, resulting in significant deviations of the electronic structure from that of the bulk.…”

“…WF values of NS films and reference bulk materials taken from the literature 6,13,14,46. The extrapolated values are labelled by *.…”