Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. Such processes have been directly visualized in the case of the model photocatalyst surface TiO 2 ð110Þ in reactions with water and molecular oxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centers and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of molecular oxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with electronically charged hydroxyl favor a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product.The rutile TiO 2 ð110Þ surface, which we use as a model photocatalytic system here, is displayed as a ball model in Fig. 1A where the reduction of one oxygen atom of O 2 ðgÞ to one bridging oxide species (O 2− b ) is accomplished by oxidation of the two Ti 3þ sites associated with O b -vac to Ti 4þ (3), on the basis of a purely ionic model. (Formal charges are written in reactions 1 and 2 to highlight the redox processes involved.)The interaction of O 2 with OH b , on the other hand, is still a matter of controversy. Following the reaction of these species at temperatures ≤240 K, water is seen to desorb at ∼310 K in temperature programmed desorption (TPD) spectra (3, 4). Henderson et al. (3) concluded that this water evolution is a consequence of the formation of oxygen adatoms (O ad ) at the surface as follows:where the two Ti 3þ species provide the two electrons necessary to reduce one oxygen atom of O 2 ðgÞ to H 2 OðgÞ (3). In stark contrast to the TPD results, previous calculations find H 2 O 2 to be by far the most stable product (5). Moreover, on the basis of these calculations, water desorption is not expected up to the highest temperature computed, 350 K (5). This discrepancy provided the initial motivation for the present work. Results and DiscussionWe use STM to provide an additional experimental test of the picture that has emerged thus far. Fig. 1B shows a surface containing both O b -vac and OH b , alongside the same surface in Fig. 1C after it was exposed to 90 Langmuirs (L) O 2 at 300 K (1 L ¼ 1.33 × 10 −6 mbar · s, 1 mbar ¼ 100 Pa). A number of small, bright spots can be seen on the Ti 5c sites (bright rows) in the latter image. The histogram of the height distribution of these bright spots, shown in Fig. 1D, indicates that these bright spots are almost entirely due to one final product.It should be noted that at lower O 2 exposures we see a number of different types of species on Ti 5c rows that are likely to arise from terminal hydroxyls (OH t ) and other metastable species such as O 2 H. These latter results ...
Organic semiconductors are studied intensively for applications in electronics and optics, and even spin-based information technology, or spintronics. Fundamental quantities in spintronics are the population relaxation time (T1) and the phase memory time (T2): T1 measures the lifetime of a classical bit, in this case embodied by a spin oriented either parallel or antiparallel to an external magnetic field, and T2 measures the corresponding lifetime of a quantum bit, encoded in the phase of the quantum state. Here we establish that these times are surprisingly long for a common, low-cost and chemically modifiable organic semiconductor, the blue pigment copper phthalocyanine, in easily processed thin-film form of the type used for device fabrication. At 5 K, a temperature reachable using inexpensive closed-cycle refrigerators, T1 and T2 are respectively 59 ms and 2.6 μs, and at 80 K, which is just above the boiling point of liquid nitrogen, they are respectively 10 μs and 1 μs, demonstrating that the performance of thin-film copper phthalocyanine is superior to that of single-molecule magnets over the same temperature range. T2 is more than two orders of magnitude greater than the duration of the spin manipulation pulses, which suggests that copper phthalocyanine holds promise for quantum information processing, and the long T1 indicates possibilities for medium-term storage of classical bits in all-organic devices on plastic substrates.
We propose a new approach to constructing gates for quantum information processing, exploiting the properties of impurities in silicon. Quantum information, embodied in electron spins bound to deep donors, is coupled via optically induced electronic excitation. Gates are manipulated by magnetic fields and optical light pulses; individual gates are addressed by exploiting spatial and spectroscopic selectivity. Such quantum gates do not rely on small energy scales for operation, so might function at or near room temperature. We show the scheme can produce the classes of gates necessary to construct a universal quantum computer.
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