We determined the interface dipoles at a number of metal-organic interfaces using ultraviolet and x-ray photoelectron spectroscopy. A linear dependence of the dipole on the metal work function is observed. This is consistent with the theory based on the charge transfer and thermodynamic equilibrium across the interface. The agreement suggests that charge transfer is one major factor in the formation of interface dipole. In addition, we find that the pushing back of the electron cloud tail that extends out of the metal surface and the permanent dipole moment of the organic molecule affect the interface dipole.
We examined the interfaces of pentacene on LiF/Au substrates as a function of LiF thickness. We found that, regardless of the thickness of LiF, upon pentacene deposition onto LiF, the pentacene vacuum level aligns with that of LiF. We also show that LiF exhibits an interface dipole when deposited onto Au and that the magnitude of the interface dipole increases as the LiF thickness increases. The change in vacuum level as a function of LiF thickness allows the Fermi level position within the band gap of pentacene to be moved from 0.5 eV above the highest occupied molecular orbital to 2.1 eV above the highest occupied molecular orbital. This produces a hole injection barrier of 0.5 eV at the pentacene/Au interface and an electron injection barrier of 0.1 eV at the pentacene/40-Å-LiF/Au interface.
We examined the interface formed by pentacene deposition onto a SiO2 substrate. We found that upon pentacene deposition onto SiO2 the pentacene vacuum level aligns with that of SiO2. We observe the immediate appearance of a measurable pentacene highest occupied molecular orbital upon deposition of as little as 2 Å of pentacene onto the SiO2 surface. This suggests that there are no chemical bonds at these interfaces. Measurements that examine the behavior of pentacene deposited onto SiO2 show ordered growth of pentacene with no sign of chemical interaction or charge transfer.
A series of primary, secondary, and tertiary amines have been examined as cosolvents with trioctylphosphine oxide (TOPO) for the synthesis of CdSe nanoclusters. Syntheses were conducted in 66 mol % hexadecylamine (HDA), dodecylamine (DDA), dioctylamine (DOA), or trioctylamine (TOA) in TOPO and the growth rates and size distributions of the resulting products compared with those obtained from the same reaction conducted in pure TOPO. DOA was found to advantageously slow the growth rate of the nanoclusters and produce material with a narrow size distribution and moderate fluorescence quantum yield. Thermal gravimetric analysis (TGA) of the products has been used to quantify the ratio of TOPO to amine on the nanocluster surface and shows that the primary amines attain the highest packing densities. STM imaging of materials synthesized in the presence of HDA or DOA and then self-assembled on nonanedithiol SAMs shows improved surface stability for the DOA synthesized products.
The electronic structures of interfaces between metals and Copper phthalocyanine (CuPc) organic films are investigated using the combination of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be directly observed by IPES and UPS simultaneously. We found that the Fermi level, EF, in the organic film can be modified by metals through charge transfer or doping. The FERMI level at the Cs/CuPc interface is observed to shift to less than 0.2 eV below the CuPc LUMO. The IPES observation is the first direct confirmation of Fermi level pinning near the LUMO in organic films. The pinning of the Fermi level close to the LUMO can be explained by electron transfer from Cs to CuPc, which is supported by the presence of a gap state in CuPc as observed with UPS. On the other hand, the Au/CuPc interface is characterized by electron transfer from CuPc to Au, resulting in a reduced HOMO intensity shown in the UPS spectra and a new feature below the LUMO shown in the IPES spectra. These observations shed new light onto the understanding of interface formation in organic semiconductor devices.
Pentacene, perylene, and sexithiophene are all materials being used in organic thin film transistors due to their relatively large mobilities. It has been suggested that the functional behavior in these devices occurs within the first few molecular layers of the organic at the interfaces between the organic and the dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene, perylene, and sexithiophene on conductors, dielectrics, and charge transfer agents and in some cases vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. There appears to be a linear relation between the interface dipole and metal workfunction with the observed dipoles ranging from a 1 eV dipole at the interface between sexithiophene and gold to a -0.46 eV dipole at the interface between pentacene and calcium. We also observed that more complex material intermixing takes place during metal on organic deposition than during organic deposition onto metal and as a result, the electronic structure of the interface differs from that of organic on metal deposition. Possible charge transfer, dipole formation and energy level bending at these interfaces will be discussed.
We have investigated the evolution of the growth front of perylene, an organic semiconductor with high carrier mobility, on glass and Au substrates grown side-by-side by vapor deposition. The films were grown with gradually increasing thickness which allowed us to examine both the spatial and temporal correlation of the surface roughness using atomic force microscopy. Our results show that perylene growth on glass and Au substrates is non-stationary. However, the instability during the growth is shown to depend largely on the substrate. A roughness exponent of 0.82 is obtained for glass and 0.84 for Au. A growth exponent of 0.21 is obtained for glass and 0.74 for Au. The results indicate the strong influence of the substrate on the film morphology and point to possible ways to control and improve it.
'/%3e%3c/defs%3e%3cpath fill='%23012169' d='M0 0h10080v5040H0z'/%3e%3cpath stroke='%23fff' stroke-width='.6' d='m0 0 6 3m0-3L0 3' clip-path='url(%23b)' transform='scale(840)'/%3e%3cpath stroke='%23e4002b' stroke-width='.4' d='m0 0 6 3m0-3L0 3' clip-path='url(%23c)' transform='scale(840)'/%3e%3cpath stroke='%23fff' stroke-width='840' d='M2520 0v2520M0 1260h5040'/%3e%3cpath stroke='%23e4002b' stroke-width='504' d='M2520 0v2520M0 1260h5040'/%3e%3cg fill='%23fff'%3e%3cuse xlink:href='%23d' x='2520' y='3780'/%3e%3cuse xlink:href='%23a' x='7560' y='4200'/%3e%3cuse xlink:href='%23a' x='6300' y='2205'/%3e%3cuse xlink:href='%23a' x='7560' y='840'/%3e%3cuse xlink:href='%23a' x='8680' y='1869'/%3e%3cuse xlink:href='%23e' x='8064' y='2730'/%3e%3c/g%3e%3c/svg%3e)
'/%3e%3cuse xlink:href='%23a' transform='rotate(23.036 .093 25.536)'/%3e%3cuse xlink:href='%23a' transform='rotate(45.87 1.273 16.18)'/%3e%3cuse xlink:href='%23a' transform='rotate(69.945 .996 12.078)'/%3e%3cuse xlink:href='%23a' transform='rotate(20.66 -19.689 31.932)'/%3e%3c/svg%3e)