Interrelation of work function and surface stability: the case of BaAl4

The relationship between the work function (Phi) and the surface stability of compounds is, to our knowledge, unknown, but very important for applications such as organic light-emitting diodes. This relation is studied using first-principles calculations on various surfaces of BaAl4. The most stable surface [Ba terminated (001)] has the lowest Phi (1.95 eV), which is lower than that of any elemental metal including Ba. Adding barium to this surface neither increases its stability nor lowers its work function. BaAl4 is also strongly bound. These results run counter to the common perception that stability and a low Phi are incompatible. Furthermore, a large anisotropy and a stable low-work-function surface are predicted for intermetallic compounds with polar surfaces.Comment: 4 pages, 5 figures, to be published in Chem. Ma

Vibrations at 3C-SiC(001)-(3 x 2) surfaces

Si-terminated 3C-SiC(001) surfaces with ($3 \times 2$) and ($2 \times 1$) reconstructions were investigated by high-resolution electron energy-loss spectroscopy (HREELS), low-energy electron diffraction (LEED) and Auger electron spectroscopy. The surfaces were prepared by subsequent annealing steps after cleaning by heating in a Si flux. At ($3 \times 2$)-reconstructed surfaces, the HREELS intensity increases while the widths of the loss lines decrease with proceeding preparation. Eventually, weak loss structures at 380 and 700 cm-1 are detected besides the strong Fuchs-Kliewer phonon loss peaks. They are attributed to surface-localized vibrations, i.e., to stretching modes of on-top Si dimers and of C-Si-C groups, respectively. The weak features vanish after exposure to atomic deuterium, but reappear after subsequent annealing. At ($2 \times 1$) reconstructed surfaces the HREELS lines are broadened and no surface-localized modes were resolved

Effect of the alkali metal content on the electronic properties of PEDOT:PSS

\u3cp\u3eThe effect of the sodium and cesium ion surface concentration on the electronic properties of spin-coated poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonic acid) films, known as PEDOT:PSS, has been studied by means of ultraviolet and X-ray photoelectron spectroscopy. The sodium and cesium concentration in the film has been varied by the addition of NaOH or CsOH to the PEDOT:PSS dispersion. Hydrogen ions of the acid PSSH are exchanged for sodium or cesium ions, resulting in the salt PSSNa or PSSCs without changing the oxidation state of PEDOT, i.e., without doping/dedoping the material. The work function changes from 5.1 to 4.0 eV with increasing alkali surface concentration. The ionization potential remains constant at 5.0 eV above 1 at% alkali metal content and coincides with the work function below 1 at%. Thus, the material changes from a semiconductor-like to a metal-like state.\u3c/p\u3

Modification of PEDOT:PSS as hole injection layer in polymer LEDs

\u3cp\u3ePoly(3,4-ethylenedioxythiophene):poly(styrenesulphonic acid) (PEDOT:PSS) is commonly used as an anode in polymer light-emitting diodes (PLED). We have studied the effect of the pH and Na\u3csup\u3e+\u3c/sup\u3e ion concentration of the aqueous PEDOT:PSS dispersion on the bulk and surface properties of spincoated films by various techniques, including UV-vis-NIR optical absorbance spectrometry, Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Ultraviolet Photoemission Spectroscopy (UPS). A pH increase by addition of NaOH modifies the PEDOT : PSS properties in a similar way as electrochemical dedoping: the IR absorbance decreases, the Raman peaks shift, sharpen and increase in intensity, and the work function decreases. Consequently, a barrier for hole injection is introduced for several classes of light-emitting polymers. We argue that the mechanism of the pH-effect is different from electrochemical dedoping, and originates from a change in the relative stability of polarons and bipolarons on the doped thiophene. The changes in the electronic properties of PEDOT:PSS point to the determining role of the counter-ion in the stabilisation of oxidised thiophene units. Polymer LEDs comprising Na\u3csup\u3e+\u3c/sup\u3e-rich, proton poor PEDOT:PSS can show lower lifetime and efficiency than the corresponding Na\u3csup\u3e+\u3c/sup\u3e-free, proton-rich devices. For light emitting polymers which suffer from the addition of sodium to the hole injecting PEDOT:PSS, the decreased lifetime hints at hole injection as limiting factor in the degradation of these PLEDs.\u3c/p\u3