In Situ Characterization of Interfaces Relevant for Efficient Photoinduced Reactions

Oliver Supplie, Matthias M. May, Sebastian Brückner, Nadzeya Brezhneva, Thomas Hannappel, Ekaterina V. Skorb
2017 Advanced Materials Interfaces  
Solar energy conversion and photo-induced bioactive sensors are representing topical scientific fields, where interfaces play a decisive role for efficient application. The key to specifically tune these interfaces is a precise knowledge of interfacial structures and their formation on the microscopic, preferably atomic scale. Gaining thorough insight into interfacial reactions, however, is particularly challenging in relevant complex chemical environment. This review introduces a spectrum of
more » ... terial systems with corresponding interfaces significant for efficient applications in energy conversion and sensor technologies. We highlight appropriate analysis techniques capable of monitoring critical physico-chemical reactions in situ during non-vacuum preparation and photoactivity studies including welldefined inorganic epitaxial reference surfaces, buried interfaces and low-defect nucleation of disjunct epitaxial materials that are analyzed during preparation in chemical vapor environment. Their surfaces are then modified and functionalized in the gaseous and liquid environment. Finally, we review even more complex coupling of inorganic stable photoactive materials with responsive soft matter for bioactivity. Interface formation, structure and/or artificial photochemical interfacial reactions are scrutinized down to the atomic scale in real time, also accounting for equilibrium versus non-equilibrium, kinetically driven processes, in order to accelerate progresses in the realization of efficient energy materials and in the exploitation of photo-induced processes at interfaces. far, the accuracy of DFT calculations is not sufficiently precise to predict RA spectra without any experimental feedback. Only in few specific cases it is possible to deduce the given atomic surface structure from experimental in situ RA spectra by comparison to theoretical ones. The P-rich InP(100) surface The P-rich InP(100) surface is an instructive example how the correlation of DFT and RAS enabled understanding the impact of the presence of H during MOVPE preparation: in situ RA spectra of both PH3 [35, 41] and tertiarybutylphosphine (TBP) [42] prepared P-rich InP (100) surfaces show a characteristic lineshape that does not occur in H-free molecular beam epitaxy (MBE) ambient. [32, 43] DFT calculations could relate the features P1 and P2 to optical transitions involving states induced by H termination of one H atom per P dimer. [32] The buckled P dimers of this "(2x1)-like" (2x2)-2D-2H reconstructed surface [32] form zig-zag lines and the buckling may flip causing a (2x2)/c(2x4) symmetry. [44, 45] While dimer buckling may occur also on non H-terminated surfaces, [46] which would cause streaked (2x1)-like low energy electron diffraction (LEED) patterns just as for the H-terminated buckled P dimers, the existence of the P-H bonds was verified experimentally by in system Fourier transform infrared spectroscopy (FTIR) studies. [47] These studies also showed that RAS allows for in situ fine tuning of the atomic order at the InP(100) surface: [47] The P-rich surface is commonly prepared by cooling under stabilization with the precursor TBP after homoepitaxial growth. When TBP supply is stopped at 300 °C, the surface is covered with excess P and precursor residuals. [48] Cycled heating to 360 °C (without precursor supply) and cooling to 300 °C (with precursor supply despite for the very last cooling step) increases the atomic order at the surface, [47] which can be observed in situ by an increased intensity of P1 along with a redshift of its energetic position. [42, 48] In situ studies during InP:adsorbate interaction revealed that the H termination strongly increases the stability of the P-rich surface against O2, yet not H2O, [49, 50] as will be discussed more detailed in section 4. The In-rich InP(100) surface Given the different lineshape of the anisotropic fingerprints of the two surface reconstructions, RAS enables to study the transformation from P-rich to In-rich surfaces in great detail: [48, 51] Heating of the P-rich surface above 370 °C without P stabilization results in enhanced P desorption and P depletion of the surface. [48] RAS peaks originating from the occurrence of P dimers vanish and the lineshape changes towards that of the In-rich surface. Fig. 7 visualizes that the peaks at and below 2.5 eV of the RA spectrum of the In-rich surface can be attributed to transitions involving surface states related to the mixed dimer reconstruction. [36, 37] The electronic orbitals for the corresponding surface states are shown in side view in Fig. 7 . Experimental verification of the microscopic origin of the spectral features Further experimental indications for the origin of the contributions to the RA spectra of P-rich and In-rich InP(100) was obtained by analyzing their temperature-dependent phonon coupling: [40] Fitting this temperature dependence with an adequate model [52] yields the renormalization energy for each anisotropic contribution. [40] For the peaks P1 and P2 as well as In3, the renormalization energy is similar to that of the InP bulk critical point energies, which implies an "intrinsic" [23] nature. Since the E1 interband transition is close to P2 and In3, it was suggested that these anisotropies likely stem from surface modified bulk transitions. [40] P1, in contrast, which is not in the vicinity of any critical interband transition, was attributed to a transition involving both bulk and surface states. [40] In1 couples only weakly to phonons and was thus attributed to pure surface state transitions. [40] Received: ((will be filled in by the editorial staff)) Revised: ((will be filled in by the editorial staff)) Published online: ((will be filled in by the editorial staff))
doi:10.1002/admi.201601118 fatcat:i2dumkhvfvht5mrxxqbxyk5r5u