Role Of Work Function Research Articles

The Role of Work Function and Band Gap in Resistive Switching Behaviour of ZnTe Thin Films

Resistive switching behavior by engineering the electrode work function and band gap of ZnTe thin films is demonstrated. The device structures Au/ZnTe/Au, Au/ZnTe/Ag, Al/ZnTe/Ag and Pt/ZnTe/Ag were fabricated. ZnTe was deposited by thermal evaporation and the stoichiometry and band gap were controlled by varying the source–substrate distance. Band gap could be varied between 1.0 eV to approximately 4.0 eV with the larger band gap being attributed to the partial oxidation of ZnTe. The transport characteristics reveal that the low-resistance state is ohmic in nature which makes a transition to Poole–Frenkel defect-mediated conductivity in the high-resistance states. The highest R off-to-R on ratio achieved is 109. Interestingly, depending on stoichiometry, both unipolar and bipolar switching can be realized.

Role of work function in field emission enhancement of Au island decorated vertically aligned ZnO nanotapers

In this report, we demonstrate significantly enhanced field emission properties of ZnO nanotapers achieved via a corrugated decoration of Au. Field emission experiments on these Au-decorated ZnO nanotapers showed emission current densities comparable to the best results in the literature. Au decoration of 5nm also reduced the effective turn-on field to ∼0.54V/μm, compared to the as grown ZnO nanotapers, which showed a turn-on field of ∼1.1V/μm. Tunneling atomic force microscopy measurements revealed a very uniform spatial emission profile in the 5nm Au decorated nanotapers, which is a basic requirement for any large scale application. We believe that metal induced mid-gap states formed at the ZnO–Au interface are responsible for the observed low turn-on field because such interface states are known to reduce the effective work function. A direct measurement of effective work function using Kelvin probe force microscopy indeed showed more than 1.1eV drop in the case of 5nm Au decorated ZnO nanotapers compared to the pristine nanotapers, supporting the above argument.

Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

AbstractThe role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (VOC) in bulk heterojunction organic photovoltaics is examined for poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and [6,6]‐phenyl‐C71 butyric acid methyl ester (PC71BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the VOC, as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J0. Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J0 and increased shunt resistance, RSH) and improved VOC. Based on these observations, we provide a set of contact design criteria for tuning the VOC in bulk heterojunction organic photovoltaics.

Role of Work Function in Electron Ejection by Metastable Atoms: Helium and Argon on (111) and (110) Tungsten

A tungsten single crystal, cut to expose the (111) and (110) planes, was employed to examine the influence of the work function on electron ejection by beams of helium or argon metastable atoms. The (111) and (110) planes have work functions which differ by approximately 1.5 eV. The electron yield of the (111) surface, with incident helium metastable atoms, is about 1.5% higher than that of the (110) surface; with argon this difference is about 4%. The ejected-electron energy distributions are essentially the same for both crystal planes, but different distributions are obtained with helium and argon. The work-function values inferred from these measurements are the same for both crystal planes, but different values are obtained depending on which metastable-atom probe is used. These results differ from what has been previously predicted, and they are qualitatively accounted for by recognizing that the resonance ionized metastable atom strongly perturbs the surface electron cloud, causing the local vacuum potential to be shifted. The shift in vacuum potential is calculated for a one-dimensional model using a pseudopotential approximation for the resonance ionized metastable atom.