Site-Selective Excitation of Ti3+ Ions in Rutile TiO2 via Anisotropic Intra-Atomic 3d → 3d Transition.

Site-selective excitation (SSE), which is usually realized by tuning the wavelength of absorbed light, is an ideal way to study bond-selective chemistry, analyze the crystal structure, investigate protein conformation, etc., eventually leading to active manipulation of desired processes. Herein, SSE has been explored in (110)-, (100)-, and (011)-faced rutile TiO2, a prototypical material in both surface science and photocatalysis fields. Using ultraviolet photoelectron spectroscopy and photon energy-, substrate orientation-, and laser polarization-dependent two-photon photoemission spectroscopy (2PPE), intra-atomic 3d → 3d transition from the split Ti3+ 3d orbitals, i.e., band gap states and excited states at ∼1.00 eV below and ∼2.40 eV above the Fermi level, respectively, has been proven for all of the samples, suggesting that it is a common property of this material. The distinct structure of rutile TiO2 results in the anisotropic 3d → 3d transitions with the transition dipole moment along the long axes ([110] and [11̅0]) of TiO6 blocking units. This anisotropy facilitates the selective excitation of Ti3+ ions in the two types of TiO6, which cannot be realized by conventional wavelength tuning, via polarization alignment of the excitation source. Discovery in this work builds the foundation for future investigation of site-selective photophysical and photochemical processes and eventually possible active manipulation in this material at the atomic level.

Noncollinear Optical Parametric Amplification of Broadband Infrared Sum Frequency Generation Vibrational Spectroscopy.

Sum-frequency generation (SFG) vibrational spectroscopy is an invaluable tool in surface science, known for its specificity to surfaces and interfaces. Despite its wide application, it is often hampered by weak signal detection. Here, we present an innovative enhancement technique of postsample amplification, using a picosecond noncollinear optical parametric amplifier (NOPA). We conducted a systematical investigation into the impact of different intensities of pump and SFG seed light, as the input signal in NOPA, and demonstrated this method on the octadecanethiol (ODT) molecules on gold films. The amplified SFG by NOPA reproduced the SFG vibrational spectra, enhanced by about 4 orders of magnitude but with broader spectral resolution due to the short pulse width of the pump light in NOPA. This study makes it possible to realize highly sensitive SFG measurements, marking a significant advancement in spectroscopic analysis techniques.

Unveiling the Intrinsic Photophysics in Quasi-Two-Dimensional Perovskites.

The two-dimensional (2D) perovskites have drawn intensive attention due to their unique stability and outstanding optoelectronic properties. However, the debate surrounding the spatial phase distribution and band alignment among different 2D phases in the quasi-2D perovskite has created complexities in understanding the carrier dynamics, hindering material and device development. In this study, we employed highly sensitive transient absorption spectroscopy to investigate the carrier dynamics of (BA)2(MA)n-1PbnI3n+1 quasi-2D Ruddlesden-Popper perovskite thin films, nominally prepared as n = 4. We observed the carrier-density-dependent electron and hole transfer dynamics between the 2D and three-dimensional (3D) phases. Under a low carrier density within the linear response range, we successfully resolved three ultrafast processes of both electron and hole transfers, spanning from hundreds of femtoseconds to several picoseconds, tens to hundreds of picoseconds, and hundreds of picoseconds to several nanoseconds, which can be attributed to lateral-epitaxial, partial-epitaxial, and disordered-interface heterostructures between 2D and 3D phases. By considering the interplay among the phase structure, band alignment, and carrier dynamics, we have proposed material synthesis strategies aimed at enhancing the carrier transport. Our results not only provide deep insights into an accurate intrinsic photophysics of quasi-2D perovskites but also inspire advancements in the practical application of these materials.

Vibronic coupling of Rhodamine 6G molecules studied by doubly resonant sum frequency generation spectroscopy with narrowband infrared and broadband visible.

Doubly resonant sum frequency generation (DR-SFG) serves as a potent characteristic technique for probing the electronic spectra and vibronic coupling of molecules on surfaces. In this study, we successfully developed a novel infrared (IR)-white light (WL) DR-SFG spectroscopy based on narrowband IR and tunable broadband WL. This novel method was employed to explore the excitation spectrum and vibronic couplings of sub-monolayer Rhodamine 6G molecules. Our findings elucidate that the xanthene skeleton vibrational modes exhibit strong coupling with the S0-S1 electronic transition. Notably, we observed not only the 0-0 transition of the S0-S1 electronic continuum but also the 0-1 transition, a first time observation in the realm of DR-SFG spectroscopy. This advanced DR-SFG spectroscopy methodology facilitates a more sensitive examination of electronic spectra and the coupling between electronic transitions and vibrational modes, heralding a significant advancement in the understanding of molecular interactions on surfaces.

Probing the Genuine Carrier Dynamics of Semiconducting Perovskites under Sunlight.

Understanding the nature of photogenerated carriers and their subsequent dynamics in semiconducting perovskites is important for the development of solar cell materials and devices. However, most ultrafast dynamic measurements on perovskite materials were conducted under high carrier densities, which likely obscures the genuine dynamics under low carrier densities in solar illumination conditions. In this study, we presented a detailed experimental study of the carrier density-dependent dynamics in hybrid lead iodide perovskites from femtosecond to microsecond using a highly sensitive transient absorption (TA) spectrometer. From the dynamic curves with low carrier density in the linear response range, we observed two fast trapping processes that occurred in less than 1 ps and tens of picoseconds, attributed to the shallow traps, and two slow decays with lifetimes of hundreds of nanoseconds and longer than 1 µs, related to the trap-assisted recombination and trapping at deep traps. Further TA measurements clearly show that PbCl2 passivation can effectively reduce both shallow and deep trap densities. These results provide insights into the intrinsic photophysics of semiconducting perovskites with direct implications for photovoltaic and optoelectronic applications under sunlight.

Valence Band of Rutile TiO2(110) Investigated by Polarized-Light-Based Angle-Resolved Photoelectron Spectroscopy.

Band structure dictates optical and electronic properties of solids and eventually the efficiency of the semiconductor based solar conversion. Compared to numerous theoretical calculations, the experimentally measured band structure of rutile TiO2, a prototypical photocatalytic material, is rare. In this work, the valence band structure of rutile TiO2(110) is measured by angle-resolved photoelectron spectroscopy using polarized extreme ultraviolet light. The effective mass of the hole, which has never been measured before, is determined to be 4.66-6.87 m0 (free electron mass) and anisotropic. The dependence of photoemission intensities on excitation light polarization is analyzed by taking into account of the parity symmetry of molecular orbitals in the blocking unit of rutile TiO2. This work reports a direct measurement of valence band structure and hole effective mass of rutile TiO2(110), which will deepen our understanding of the electronic structure and charge carrier properties of the model material and provide reference data for future theoretical calculations.

Anisotropic d-d Transition in Rutile TiO2.

The band gap state of TiO2, which is dominated by Ti3+ 3d character, is of great relevance to light absorption, electron trapping, charge recombination, and conduction band structure. Despite the importance, the explanation of the excitation from this state is controversial. To this end, the electronic structures of TiO2(110) and TiO2(011)-(2 × 1) have been systematically measured with two-photon photoemission spectroscopy. The results reveal the anisotropic nature of the electronic structure in rutile TiO2 at seemingly equivalent directions of [110] and [11̅0], the long axes of the TiO6 blocking unit. Although the resonant energy of these two d-d transitions is identical, the energy levels are systematically shifted by 0.1 eV. We propose this anisotropy originates from the broken symmetry of the rutile TiO2 crystals caused by the surface. The proposed asymmetry-caused electronic structure anisotropy could be generalized to other similar materials and may affect associated catalytic properties. This work provides an important benchmark for related calculations.

Efficient generation of narrowband picosecond pulses from a femtosecond laser.

In some applications of broadband ultrafast spectroscopy, such as surface sum frequency generation vibrational spectroscopy, femtosecond stimulated Raman spectroscopy (SRS), and coherent anti-Stokes Raman spectroscopy, a narrowband picosecond pulse is required to obtain a high spectral resolution. Here, we present a method to generate narrowband picosecond second harmonic (SH) and fundamental frequency (FF) pulses with high-conversion efficiency from a Ti:sapphire femtosecond laser amplifier. The narrowband picosecond SH pulse was generated based on the group velocity mismatch between the SH and FF pulses in a nonlinear crystal of ß-barium borate (BBO). The small SH nonlinear optical coefficient was optimized by changing the azimuth angle of a thick BBO crystal, successfully avoiding the saturation effect in the SH generation process. The SH pulse was then used to pump an optical parametric amplifier to efficiently amplify the narrowband FF seed pulse, which was obtained with an etalon by spectrally filtering the output from the femtosecond laser amplifier. Dual-wavelength output, which could be very useful in femtosecond SRS, was also realized.

Ultrahigh sensitive transient absorption spectrometer.

Transient absorption (TA) spectroscopy is considered as a powerful technique that reflects the ultrafast dynamics of photogenerated carriers in photoelectric and photocatalysis materials. However, limited by its sensitivity, the photogenerated carrier density in TA measurements of solar energy materials is usually much higher than that in the real working condition. Here, we present a combination of kHz macro-pulse and MHz micro-pulse technique for an ultrahigh sensitive TA spectrometer, which improves the sensitivity to the 10-7 level of ΔOD. It enables us to study ultrafast carrier dynamics pumped by very low power, which can avoid the influence of many-body interactions and the nonlinear effect associated with high carrier density. This work provides a novel TA method with ultrahigh sensitivity, which will play an important role in investigating the carrier dynamics of semiconductors in the working condition.

Ultralong UV/mechano-excited room temperature phosphorescence from purely organic cluster excitons.

Purely organic room temperature phosphorescence (RTP) has attracted wide attention recently due to its various application potentials. However, ultralong RTP (URTP) with high efficiency is still rarely achieved. Herein, by dissolving 1,8-naphthalic anhydride in certain organic solid hosts, URTP with a lifetime of over 600 ms and overall quantum yield of over 20% is realized. Meanwhile, the URTP can also be achieved by mechanical excitation when the host is mechanoluminescent. Femtosecond transient absorption studies reveal that intersystem crossing of the host is accelerated substantially in the presence of a trace amount of 1,8-naphthalic anhydride. Accordingly, we propose that a cluster exciton spanning the host and guest forms as a transient state before the guest acts as an energy trap for the RTP state. The cluster exciton model proposed here is expected to help expand the varieties of purely organic URTP materials based on an advanced understanding of guest/host combinations.

Single Molecule Photocatalysis on TiO2 Surfaces.

Heterogeneous photocatalysis has been widely applied in various fields, such as photovoltaic cell, solar water splitting, photocatalytic pollutant degradation, and so on. Therefore, the reaction mechanisms involved in these important photocatalytic processes, especially in TiO2 photocatalysis, have been extensively investigated by various surface science techniques in the past decade. In this review, we highlight the recent progress that provides fundamental insights into TiO2 photocatalysis through direct tracking the evolution of single molecule photochemistry on TiO2 single crystal surfaces using a combination of scanning tunneling microscopy (STM) and other surface science techniques. Insight into the structures of various TiO2 surfaces is discussed first, which provides a basic concept on TiO2. Afterward, the details of the single molecule photocatalysis of several important molecules (water, alcohols, and aldehydes) on the model TiO2 surfaces are presented, which are trying to probe bond cleavages and the roles of adsorption sties and adsorption states in TiO2 photocatalysis step-by-step. Last, challenges and opportunities in single molecule photocatalysis on TiO2 are discussed briefly.

Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges.

Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO2 photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO2 photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO2 photocatalysis at a molecular level is highlighted. Insights into the structures of TiO2 and the basic principles of TiO2 photocatalysis are discussed first, which provides the basic concepts of TiO2 photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO2 surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO2 photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO2 photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.

Flexible high-resolution broadband sum-frequency generation vibrational spectroscopy for intrinsic spectral line widths.

The difficulty in achieving high spectral resolution and accurate line shape in sum-frequency generation vibrational spectroscopy (SFG-VS) has restricted its use in applications requiring precise detection and quantitative analysis. Recently, the development of high-resolution broadband sum-frequency generation vibrational spectroscopy (HR-BB-SFG-VS) with sub-wavenumber resolution generated by synchronizing two independent amplifier lasers have opened new opportunities for probing an intrinsic SFG response. Here, we present a new flexible approach to achieve HR-BB-SFG-VS. In this system, two regeneration amplifiers shared the same oscillator laser as the seed, and a time-asymmetric visible pulse with a nearly Lorentzian line shape filtered by an etalon was used to overlap with a femtosecond broadband infrared pulse. This Lorentzian line shape of the visible pulse can greatly simplify the spectral fitting and analysis. We also demonstrated that the single-sided long visible pulse provided both high spectral resolution (1.4 cm-1) and effective suppression of the non-resonant background by detuning the time delay between visible and infrared pulses in SFG-VS measurements. With this new SFG setup, a pair of spectral splittings by 3.1 ± 0.7 and 3 ± 0.2 cm-1 for the symmetric and antisymmetric stretching of the CH3 group was resolved at the CH3CN/TiO2(110) surface, which are tentatively attributed to two different orientational methyl groups. These technological advancements can help broaden the applications of HR-BB-SFG-VS and provide solid ground for a better understanding of complex molecular structures and dynamics at interfaces.

Electronic structure and photoabsorption of Ti3+ ions in reduced anatase and rutile TiO2.

We have used two-photon photoemission (2PPE) spectroscopy and first-principles density functional theory calculations to investigate the electronic structure and photoabsorption of the reduced anatase TiO2(101) and rutile TiO2(110) surfaces. 2PPE measurements on anatase (101) show an excited resonance induced by reduced Ti3+ species centered around 2.5 eV above the Fermi level (EF). While this state is similar to that observed on the rutile (110) surface, the intensity of the 2PPE peak is much weaker. The computed oscillator strengths of the transitions from the occupied gap states to the empty states in the conduction band show peaks between 2.0 and 3.0 eV above the conduction band minimum (CBM) on both surfaces, confirming the presence of empty Ti3+ resonances at these energies. Although the crystal field environment of Ti ions is octahedral in both rutile and anatase, Ti3+ ions exhibit distinct d orbital splittings due to different distortions of the TiO6 units. This affects the directions of the transition dipoles from the gap states to the conduction band, explaining the polarization dependence of the 2PPE signal in the two materials. Our results also show that the Ti3+ induced states in the band gap are shallower in anatase than in rutile. The d → d transitions from the occupied gap states to the empty Ti3+ excited states in anatase can occur at energies well below 3 eV, consistent with the observed visible-light photocatalytic activity of Ti3+ self-doped anatase.

Observation and Manipulation of Visible Edge Plasmons in Bi2Te3 Nanoplates.

Noble metals, like Ag and Au, are the most intensively studied plasmonic materials in the visible range. Plasmons in semiconductors, however, are usually believed to be in the infrared wavelength region due to the intrinsic low carrier concentrations. Herein, we observe the edge plasmon modes of Bi2Te3, a narrow-band gap semiconductor, in the visible spectral range using photoemission electron microscopy (PEEM). The Bi2Te3 nanoplates excited by 400 nm femtosecond laser pulses exhibit strong photoemission intensities along the edges, which follow a cos4 dependence on the polarization state of incident beam. Because of the phase retardation effect, plasmonic response along different edges can be selectively exited. The thickness-dependent photoemission intensities exclude the spin-orbit induced surface states as the origin of these plasmonic modes. Instead, we propose that the interband transition-induced nonequilibrium carriers might play a key role. Our results not only experimentally demonstrate the possibility of visible plasmons in semiconducting materials but also open up a new avenue for exploring the optical properties of topological insulator materials using PEEM.

Elementary Chemical Reactions in Surface Photocatalysis.

Photocatalytic hydrogen evolution and organic degradation on oxide materials have been extensively investigated in the last two decades. Great efforts have been dedicated to the study of photocatalytic reaction mechanisms of a variety of molecules on TiO2 surfaces by using surface science methods under ultra-high vacuum (UHV) conditions, providing fundamental understanding of surface chemical reactions in photocatalysis. In this review, we summarize the recent progress in the study of photocatalysis of several important species (water, methanol, and aldehydes) on different TiO2 surfaces. The results of these studies have provided us deep insights into the elementary processes of surface photocatalysis and stimulated a new frontier of research in this area. Based on the results of these studies, a new dynamics-based photocatalysis model is also discussed.

Macroscopic Wires from Fluorophore-Quencher Dyads with Long-Lived Blue Emission.

We report the formation of macroscopic wires up to centimeters in length from a series of structurally flexible, covalently tethered small-molecular fluorophore-quencher dyads (FQDs, average MW = 425 Da), comprised of carbazole, melatonin, and cyanobenzoate moieties. These FQDs are nonemissive in organic solutions but become moderately to highly luminescent (ΦF = 0.037-0.39) upon formation of wires with emission maxima in the blue region (446-483 nm). The blue photoluminescence (PL) is ascribed to a combination of singlet charge transfer, localized triplet state, and possibly delayed fluorescence emissions with intrinsic luminescence lifetimes ranging from 0.228 to 21333 µs, based on luminescence, transient absorption measurements, X-ray diffraction, and calculations.

Photocatalytic chemistry of methanol on rutile TiO2(011)-(2 × 1).

Photocatalytic chemistry of methanol on the reconstructed rutile TiO2(011)-(2 × 1) surface upon 266 nm and 400 nm light excitation has been investigated quantitatively using the post-irradiation temperature-programmed desorption (TPD) method. Photochemical products such as formaldehyde, methyl formate and water, which result from the recombination of surface bridging hydroxyls through the abstraction of lattice oxygen atoms, have been identified under both 266 nm and 400 nm light irradiation. However, ethylene is detected only under 266 nm light irradiation. Through an analogy experiment, ethylene production is attributed to the photochemistry and the following thermochemistry of formaldehyde. The absence of the ethylene signal under 400 nm light is consistent with the significantly lower conversion at this wavelength compared with 266 nm. The photocatalytic reaction rate of methanol is also wavelength dependent. Possible reasons for the photon energy dependent phenomena have been discussed. This work not only provides a detailed characterization of the photochemistry of methanol on the rutile TiO2(011)-(2 × 1) surface, but also indicates the importance of photon energy in the photochemistry on TiO2 surfaces.

Elementary photocatalytic chemistry on TiO2 surfaces.

Photocatalytic hydrogen production and pollutant degradation provided both great opportunities and challenges in the field of sustainable energy and environmental science. Over the past few decades, we have witnessed fast growing interest and efforts in developing new photocatalysts, improving catalytic efficiency and exploring the reaction mechanism at the atomic and molecular levels. Owing to its relatively high efficiency, nontoxicity, low cost and high stability, TiO2 becomes one of the most extensively investigated metal oxides in semiconductor photocatalysis. Fundamental studies on well characterized single crystals using ultrahigh vacuum based surface science techniques could provide key microscopic insight into the underlying mechanism of photocatalysis. In this review, we have summarized recent progress in the photocatalytic chemistry of hydrogen, water, oxygen, carbon monoxide, alcohols, aldehydes, ketones and carboxylic acids on TiO2 surfaces. We focused this review mainly on the rutile TiO2(110) surface, but some results on the rutile TiO2(011), anatase TiO2(101) and (001) surfaces are also discussed. These studies provided fundamental insights into surface photocatalysis as well as stimulated new investigations in this exciting field. At the end of this review, we have discussed how these studies can help us to develop new photocatalysis models.

Coverage Dependence of Methanol Dissociation on TiO2(110).

Although the photochemistry of methanol on TiO2(110) has been widely investigated as a prototypical model of the photocatalytic reaction of organic molecules, the most fundamental question of the adsorption state of methanol on TiO2(110) is still unclear. We have investigated the adsorption of methanol on TiO2(110) using sum frequency generation vibrational spectroscopy (SFG-VS) and density functional theory (DFT) calculations. The SFG results indicate the dissociation of methanol is highly dependent on the coverage. The DFT calculations suggest that the methanol prefers the partially dissociated structure at low coverage, whereas the second layer methanol, which is hydrogen-bonded to the bridge-bonded oxygen site, largely blocks the dissociation of the first layer methanol. Our results not only resolves a long-standing debate regarding the adsorption state of methanol on TiO2(110) but also provides a detailed insight into the adsorption structure and sheds light on the photochemistry on this surface at the molecular level.