Simultaneous solution-based generation and characterization of crystalline bismuth thin film by femtosecond laser spectroscopy

Among heavy metals and semimetals, bismuth (Bi) is considered to be a relatively green and environmentally benign element. With growing environmental concerns and the need for greener reagents, interest in Bi and its compounds has steadily increased in the past decade for wide applications in semiconductors, medicine, photocatalysis, gas sensing, optical coating, radiation detection, and thermoelectrics.^1–3 In particular, triphenyl bismuth (Ph₃Bi) has been used as an X-ray contrast agent in orthopedic bone cements,⁴ additives for plastics in dentistry,⁵ and in medical implants and sutures.⁶ From the molecular perspective, this compound contains photolabile Bi–C bonds and is highly soluble in many organic solvents. As a result, the photochemistry of Ph₃Bi can be studied in solution as a precursor to Bi-centered radicals and Bi-containing solids.^7,8 Such photochemistry is also fundamentally relevant to the emerging area of metal oxide photoresists, which now exhibit the highest patterning performance among all known extreme ultraviolet (EUV) materials.⁹ 

It is known that Ph₃Bi has strong absorption in the UV and experiences homolytic bond cleavage under UV irradiation.^7,10,11 The timescale for relevant radical formation, however, remains unclear. Such studies are challenging because chemical bond rupture could occur on the molecular timescale down to the femtosecond (fs) regime. Therefore, ultrafast spectroscopic methods are needed to study excited state dynamics of such metal-organic complexes in solution,^12–14 particularly regarding the photoinduced metal-to-ligand charge transfer (MLCT) process or metal-ligand bond dissociation. In this work, we study the photochemistry of Ph₃Bi as a model system for metal-carbon bond breaking. We generate a thin film of Bi directly from Ph₃Bi in methanol solution via fs-laser-induced photolysis and crystallization without use of a chemical reductant. We simultaneously characterize the nascent Bi thin film by analysing a dominant optical phonon mode at ∼90 cm⁻¹ from spectral oscillations of fs transient absorption. The spectral data with characteristic picosecond (ps) time constants shed light on the photochemical scission of Ph₃Bi in solution, revealing the formation of an intermediate electronic state on the ∼4 ps timescale. This in-situ methodology thus provides unique insights into Bi–C bond weakening/breaking events and the transition from molecule to condensed solid.

Our fs laser system consists of a mode-locked Ti:sapphire oscillator (Mantis-5) and regenerative amplifier (Legend Elite-USP-1K-HE, Coherent), providing the fundamental pulse (FP) at 800 nm with 35 fs pulse duration, 4.1 mJ pulse energy at 1 kHz repetition rate. Part of the FP passes through a 1-mm-thick type-I beta barium borate (BBO) crystal for second harmonic generation (SHG) to obtain the 400 nm pulse. A calcite plate provides compensation for group velocity delay, and a zero-order dual waveplate (λ/2 at 800 nm and λ at 400 nm) is subsequently used to achieve parallel polarization of the 800 and 400 nm pulses. The FP residual and SHG pulses are directed through another 1-mm-thick type-I BBO to generate the third-harmonic pulse at 267 nm, which is selected with a dichroic mirror to act as the UV pump with ∼0.5 μJ pulse energy [Fig. 1].

Another portion of FP is used to generate either a 400-nm pulse via SHG in BBO or a 267-nm pulse via the broadband up-converted multicolor array technology in a 1-mm-thick calcite crystal plate.^15,16 The probe pulse energy is set at ∼100 nJ. A high-resolution motor-controlled delay stage (07EAS504, CVI Melles Griot) is used to tune the time delay between the s-polarized pump and probe pulses, which are focused onto the sample solution using an f = 10 cm lens or UV-enhanced Al-coated concave mirror with ca. 0.2-mm beam diameter at the focal point. The 267-nm probe past the sample is selected through a UV bandpass filter (FGUV11, Thorlabs), then focused and measured by a Si-biased photodiode detector. The time-dependent signal is collected via a lock-in amplifier synchronized to the optical chopper at 500 Hz. For the 400-nm probe, we recollimate the beam and focus it into the spectrograph (MS127i, Oriel) with a 600-grooves/mm, 400-nm blaze grating. The dispersed signal is collected by a CCD array camera (Princeton Instruments, PIXIS 100F) that is synchronized with the laser source to achieve shot-to-shot spectral acquisition. A typical pump-probe scan with enough time delay points and signal averaging takes ∼10 min. Ph₃Bi was purchased from Alfa Aesar and used without further purification. High performance liquid chromatography (HPLC)-grade methanol (EMD Millipore) was dried by standard procedures and freshly distilled prior to use. All experiments are carried out at atmospheric pressure (1 atm) and room temperature (72 °F). To aid the ground-state Raman mode assignment, Ph₃Bi is computed in Gaussian at the restricted Hartree-Fock (RHF)/LANL2DZ level with IEFPCM (integral equation formalism polarizable continuum model)-methanol to include the solution effect on the vibrational normal modes (with the frequency scaling factor of 0.96).¹⁷ 

We first irradiate 0.25M Ph₃Bi in methanol solution in a 1-mm-pathlength quartz cell (Spectrosil 1-Q-1, Starna Cells) with 267-nm pump (ca. 2 μJ/pulse, 5-mm beam diameter) for 20 min. A black thin film forms on the inner side of the front surface (facing incident light) of the sample cuvette, while the solution remains clear and colorless [Fig. 1 bottom]. Interestingly, after ∼30 min, if we stop the UV irradiation, this black film becomes invisible. Alternatively, if we open the cell to air, the film rapidly becomes visibly transparent, making it difficult to further characterize the thin film. Given that the black film likely consists of Bi metal as the dominant species (see below), the observed film “disappearance” can be explained as Bi being oxidized to form bismuth oxide (Bi₂O₃).⁷ Notably, in comparison to pulsed-laser deposition of bismuth thin film in vacuum chamber,^18,19 our solution-based crystallization method is much simpler with less energy footprint.¹⁶ For instance, our pump-laser fluence at ∼1 × 10⁻⁵ J/cm² is much lower than previously used values^18–20 above ∼1 mJ/cm² mainly because we use a pump pulse with fs time duration.

To substantiate the merit of fs-laser-induced Bi crystallization, we performed an airtight control experiment. In a 50-ml Schlenk flask, 300-mg Ph₃Bi was dissolved in 20-ml dry methanol (sample concentration of ≈34 mM), stirred for 5 min under an atmosphere of Argon, and irradiated by a mercury UV lamp (Porta-Cure 1000, American Ultraviolet, output wavelength of 185–400 nm, non-focused) at 125-W/in. setting for 75 min. After 15 min, the colorless solution changed to light yellow; after 60 min, it became dark brown. The solvent was removed in vacuo to yield a black powder, which mainly contains graphitic carbon and an insignificant amount of elemental Bi as revealed by XRD analysis.²¹ Decreasing the Ph₃Bi concentration to 1 mM produces a light yellow solution without formation of black powder even after 120 min under the same condition. Changing the flask to the 1-mm-thick quartz cuvette makes no clear difference. It shows that the UV lamp cannot effectively generate Bi from Ph₃Bi solution, which could be due to much less excitation energy density, more heat-induced degradation, and/or additional photochemistry driven by wavelengths in the UV lamp spectrum that are not in the ultrafast laser bandwidth.

In the UV pump-probe setup with 267 nm pulses, the time-resolved probe transmittance on 1-mM Ph₃Bi in methanol solution as a function of irradiation time is presented in Fig. 2. The initial dip in transmittance [Fig. 2(a)] arises from two-photon absorption of methanol, reflecting the cross correlation between the pump and probe pulses (i.e., ∼100 fs instrument response time). This negative peak diminishes with time, which is consistent with the production and crystallization of Bi along the beampath; a control experiment with pure methanol shows the negative peak unchanged with irradiation time. The Bi film formed on the cuvette absorbs at 267 nm, so later datasets develop a positive feature after ∼100 fs that is indicative of electronic ground-state bleaching of Bi crystals.²² After ∼30 min, a clear exponential decay of the positive ΔT with oscillations appears with the time constant of ∼1 ps [Fig. 2(b)]. By subtracting the fitted curve from the time-resolved ΔT trace, the oscillatory residual is obtained [Fig. 2(c)]. Using fast Fourier transform (FFT), the underlying coherent modulation mode is retrieved at ∼90 cm⁻¹, which matches well with the A_1g phonon mode of crystalline Bi [Fig. 2(d)].^20,23,24 The phonon lifetime is ∼500 fs by least-squares fitting the oscillatory trace to a damped sine wave.^24,25 This result indicates that fs-UV pump pulse generates crystalline Bi on the cuvette inner surface, while the transient coherent optical phonon mode is impulsively excited and detected by the pump and probe pulses, respectively.^26–28 We conjecture that the focused fs-pump has sufficient peak density to trigger photolysis of solvated Ph₃Bi to generate crystalline Bi. After a certain amount of film is formed, the pump pulse induces A_1g coherent phonons in the Bi nanocrystal film that modulate its ground-state bleaching signal. The aforementioned phonon decay on the sub-ps timescale is mainly due to scattering with the incoherent population elements including thermal phonons, lattice defects, and electrons.^20,25 The observed ∼1-ps exponential decay in Fig. 2(b) corresponds to the excited-state population relaxation time of Bi crystal.²⁰ 

In order to resolve the phonon dynamics with higher frequency resolution, we further analyze the transient absorption data using discrete Fourier transform (DiFT) without zero padding. Fig. 3(b) shows that the phonon mode blueshifts within ∼30 min, i.e., from ∼86 to 98 cm⁻¹. This result is consistent with the formation and growth of the thin film, because it is observed that the Raman-active A_1g mode undergoes a frequency blueshift with increasing thickness and stiffening of the nanocrystal film.^29,30 Meanwhile, because Ph₃Bi absorbs in the UV, we implement tunable femtosecond stimulated Raman spectroscopy (FSRS) using the ps 400-nm Raman pump via a home-built second harmonic bandwidth compressor³¹ and the first sideband of sum-frequency-generation-based broadband up-converted multicolor array (SFG-BUMA)^32,33 as Raman probe to characterize the Ph₃Bi precursor. Fig. 3(a) presents the anti-Stokes FSRS data with pre-resonance Raman enhancement.^33,34 A number of Raman modes are clearly resolved within a ∼1600 cm⁻¹ spectral window,³⁵ suggesting that the Franck-Condon region of photoexcited Ph₃Bi involves significant benzene ring motions and some degree of Bi–C stretching on the sub-ps timescale.³⁶ Notably, we did not observe a 90 cm⁻¹ mode in the ground-state FSRS data of Ph₃Bi solution [Fig. 3(a)], corroborating that the phonon mode is associated with the Bi thin film [Fig. 3(b)].

To provide deeper insights into a different region of the multidimensional potential energy surface of Ph₃Bi in the photoexcited state, we use a 400-nm probe with the 267-nm pump. The transient electronic dynamics differ between datasets with increasing irradiation time, except that a dominant excited-state absorption feature, i.e., positive ΔOD, with a 4.0 ± 0.6 ps exponential decay constant is observed [Fig. 4 and inset (b)]. The time-resolved data trace after ∼30 min UV irradiation exhibits an oscillation of ∼350 fs period superimposed on the 4-ps decay [Fig. 4 inset (a)]. The phonon lifetime from least-squares fit of the oscillatory component is ∼1 ps, which is consistent with the characteristic damping time of impulsively excited coherent phonons to return to unperturbed positions.^20,27 After the initial decay, the first dataset collected immediately following irradiation (0 min) shows an additional ∼165 ps decay (30 wt. %) that could be related to further bond breaking events. The subsequent negative ΔOD signal recovers on the nanosecond time scale (beyond our current experimental time window of ∼600 ps), which is indicative of stimulated emission from an intermediate state. In contrast, following 30 min of UV irradiation, the transient data exhibit complex dynamics, wherein after reaching the minimum at ∼20 ps, the negative ΔOD experiences a slow rise with nanosecond time constant [see Fig. 4].

This unique series of transient absorption data collected in situ during Bi formation provides mechanistic insights into the Ph₃Bi photochemistry in solution. At 0 min, the fs-UV pump pulse excites Ph₃Bi molecules from ground state (S₀) to first singlet excited state (S₁, MLCT), which can further go to a higher lying excited state, e.g., S₂ or S_n. The 4-ps exponential decay of Ph₃Bi excited-state absorption signal reflects the S₁ state relaxation. If all the S₁ population returns to S₀ on the ps timescale, the transient absorption signal would diminish to zero, but we observe an evolving negative signal. The least-squares fitting reveals a ∼2 ns recovery process that likely arises from stimulated emission of an intermediate state (I₁) after photoexcited Ph₃Bi navigates out of the Franck-Condon region toward the photolytic reaction state [Fig. 4, right panel]. Before all three Bi–C bonds dissociate to form Bi, some reactive Ph₂Bi· or PhBi: radical species may form in a multiquantum process without dominant intramolecular electron transfer.^12,37 After ∼30 min of UV irradiation, the nascent crystalline Bi absorbs the 400 nm probe to some degree²² and contributes the ∼1 ps decay (∼45 wt. %) of ground-state bleaching [Fig. 2(b)], which is coupled to stimulated emission of the I₁ state (∼2 ns decay, 55 wt. %), leading to the observed complex dynamics. Notably, the observation that the ∼90 cm⁻¹ oscillation associated with impulsively excited A_1g coherent phonon motions from the Bi film is pronounced in the 30 min trace [Fig. 4 inset (a)] further substantiates this mechanism. Furthermore, after checking the 400-nm probe counts on CCD camera for each pump-probe scan, we note that at ∼30, 60, and 90 min the transmitted probe intensity decreases by ∼74%, 90%, and 93%, respectively. Since the penetration depth of crystalline Bi upon absorbing 400-nm light (∼3.1 eV) is ∼30 nm (Ref. 24) while Ph₃Bi in methanol solution does not absorb at 400 nm, we estimate that the thickness of the nascent crystalline Bi film is ∼40, 70, and 80 nm after 30, 60, and 90 min of UV (267 nm) irradiation, respectively, corroborating the observed phonon frequency blueshift in Fig. 3(b).

To elucidate the ultrafast structural dynamics of this photochemical reaction, tunable FSRS with a UV excitation pulse is needed. The ground-state Raman measurement using SFG-BUMA as Raman probe pulse was performed,³³ and three peaks of solvated Ph₃Bi stand out in Fig. 3(a): 206 cm⁻¹, Bi–C stretching and Ph₃Bi symmetric stretching; 648 cm⁻¹, benzene ring asymmetric in-plane deformation; and 1002 cm⁻¹, benzene ring breathing motion.³⁸ Following photoexcitation, the time-resolved FSRS data of Ph₃Bi in solution will expose transient atomic motions by tracking multiple vibrational mode frequencies and intensities^35,36 which reveal molecular structural evolution at the chemical bond level beyond the electronic domain [Figs. 2 and 4]. We are in the process of tuning the Raman pump to enhance excited-state FSRS features^31,33 particularly within the first ∼10 ps [see Fig. 4, vertical dashed line] and unravel the initial photolysis reaction coordinate that leads to the metal thin film.

In conclusion, we use fs 267-nm pump pulse to generate a crystalline Bi thin film directly from Ph₃Bi in methanol solution and simultaneously monitor the non-equilibrium reactant consumption and product formation with a time-delayed fs-probe pulse either at 267 or at 400 nm. We reveal a ∼4 ps time constant associated with the excited-state relaxation of Ph₃Bi, which likely involves an intermediate state with weakened and/or broken Bi–C bonds. After a certain thickness (∼40 nm) of Bi film is generated, the coherent A_1g phonon mode at ∼90 cm⁻¹ of crystalline Bi is detected in the fs transient absorption and transmittance traces, inferring the dominant electron-lattice coupling in the functional crystalline material. As the nascent Bi film thickens (up to ∼80 nm) with time, a blueshift of the totally symmetric phonon mode is observed. Our setup offers a convenient and viable approach to use fs-laser pulses to generate crystalline metallic thin films from inexpensive solution precursors while offering the opportunity to simultaneously characterize in real time photochemical reaction pathways in electronic and vibrational domains.

This work was supported by the Oregon State University Faculty Startup Research Grant and Research Equipment Reserve Fund (C.F.) and the National Science Foundation (NSF) CAREER award (CHE-1455353, C.F.). We also acknowledge funding from the NSF Center for Sustainable Materials Chemistry (CHE-1102637, L.Z., S.S., and D.A.K.).