Jacob M.
Garcia
ab and
Scott G.
Sayres
*ab
aSchool of Molecular Sciences, Arizona State University, Tempe, AZ 85287, USA. E-mail: Scott.Sayres@asu.edu
bBiodesign Center for Applied Structural Discovery, Arizona State University, Tempe, AZ 85287, USA
First published on 11th February 2022
The ultrafast electronic relaxation dynamics of neutral nickel oxide clusters were investigated with femtosecond pump–probe spectroscopy and supported with theoretical calculations to reveal that their excited state lifetimes are strongly dependent on the nature of the electronic transition. Absorption of a UV photon produces short-lived (lifetime ∼ 110 fs) dynamics in stoichiometric (NiO)n clusters (n < 6) that are attributed to a ligand to metal charge transfer (LMCT) and produces metallic-like electron–electron scattering. Oxygen vacancies introduce excitations with Ni-3d → Ni-4s and 3d → 4p character, which increases the lifetimes of the sub-picosecond response by up to 80% and enables the formation of long-lived (lifetimes >2.5 ps) states. The atomic precision and tunability of gas phase clusters are employed to highlight a unique reliance on the Ni orbital contributions to the photoexcited lifetimes, providing new insights to the analogous band edge excitation dynamics of strongly correlated bulk-scale NiO materials.
Ni has the lowest excitation energy of the first-row transition metal elements and an open 3d subshell, providing an extremely rich density of states. This enables electron–electron (e–e) scattering to be a prominent relaxation mechanism in bulk NiO, leading to excited state dynamics as short as 10s of fs7,10 and polaron formation on the sub-ps timescale.11 The d8 ground state electron configuration of NiO formally consists of fully occupied t2g and half-filled eg orbitals, with strong hybridization between the 2p and 3d bands.7 The d-shell of Ni is the most compact of all transition metals,12 enabling NiO to contain signatures of both localized atomic-like states and band-like dispersive electronic states. NiO is classified as an intermediate charge-transfer insulator, suggesting that the charge transfer energy gap (Δ) between the O-2p band and the unoccupied Hubbard band, is smaller than the weakly screened Coulomb interaction (Hubbard U),7 thereby leaving O-2p bands in the energy range of the occupied Hubbard band. Although the commonly reported band gap of NiO is 4.1–4.3 eV,8,9 the onset of its photoabsorption has been recorded as low as 3.1 eV.13 This lower energy feature is often ignored, but has been identified as excitation between the valence Ni-3d and the Ni-4s character of the conduction band minima.13 Detailed calculations provide mounting support for the importance of such s character states, which are more delocalized.14,15 This challenges conventional knowledge which considers only localized 3d (eg) states in the conduction band edge and suggests that the photodynamics of NiO materials are influenced by the involvement of different electronic orbitals.
Sub-nanometer sized clusters are the ideal venue for identifying the electronic and structural factors that govern carrier dynamics and related excited state lifetimes.16–18 Their electronic properties change with the addition or subtraction of a single atom and their finite size is completely addressable by density functional theory (DFT), enabling detailed insights about the role of defect sites in bulk-scale reactions. Nickel oxide clusters are among the least understood transition metal oxides and are also the most intriguing because they contain a high number of energetically competitive structural isomers and spin configurations. Here, femtosecond pump–probe spectroscopy coupled with theoretical calculations are used to show a unique reliance on the Ni orbital contributions to the photoexcited neutral nickel oxide cluster lifetimes, and by extension allow for a deeper understanding of the bulk-scale photoexcitation. Specifically, we show that the electron transfer from a O-2p orbital to a Ni-3d orbital, commonly referred to as ligand-to-metal charge transfer (LMCT) excitation,7,15 undergoes rapid relaxation through e–e scattering. The introduction of O vacancies enables Ni-3d → Ni-4s transitions and Ni-3d → Ni-4p transitions that exhibit delocalized carriers and slower relaxation dynamics. We demonstrate these intriguing effects by adjusting the electronic structure of clusters with atomic precision.
The excited state transient signals of neutral nickel oxide clusters are recorded by scanning the optical delay of the probe beam with respect to the pump pulse and tracking the change of intensity from each ion signal. The ion signal is proportional to the excited state population as it decays in time. The 800 nm beam was measured through autocorrelation to be <35 fs. The instrumental response function (Gaussian function) is measured to be < 50 fs (FWHM) using the enhancement of non-resonant ionization signal of O2 that matches the cross correlation. The maximum intensity of the cluster signals is recorded ∼50 fs after the temporal overlap of the two laser pulses (time zero). This delay onset of cluster signal reveals that the signal is dominated by direct ionization through a resonant excitation. The transient ion signal is a convolution of the molecular response and the cross-correlation of the two laser beams and therefore the maximum of the ion signal exhibits a temporal shift proportional to the lifetime of the cluster. An exponential decay function and plateau function are used to fit all cluster transients, described in detail previously.16,17 These are both convoluted with the Gaussian instrumental response function, where the exponential decay function accounts for the fast relaxation in transient signals associated with an intermediate metastable state of a neutral cluster that decays with the measured lifetime (τ) and the plateau function represents a stable long-lived (>2.5 ps) excited state that survives longer than the experiment.
Although the cluster distribution may contain several isomers, especially for larger clusters, the lowest energy configuration is likely dominant and can provide insight to the excited state dynamics. Thus, the lowest energy spin configuration and geometries were used as input for single point time dependent-density functional theory (TD-DFT) calculations to account for the excited state characteristics. A sufficient number of excited states are included for each cluster to exceed the pump photon energy (400 nm = 3.1 eV). An excited state population analysis was performed to determine the elemental contributions to the excited state nearest 3.1 eV. The C-squared population analysis (CSPA) method disregards the fact that basis functions may overlap, and simply defines the contribution, cai, of a particular atomic orbital, a, to a particular molecular orbital, i, as the square of the molecular orbital coefficient normalized to the square of all atomic orbital coefficients, k, as:
(1) |
Fig. 1 Mass spectrum of neutral nickel oxide clusters following photoionization of the pump–probe pulse at temporal overlap. The inset is a zoomed in to show the distribution of the larger clusters. |
The transients of the (NiO)n clusters (n = 1–5) all show a similar fast relaxation of ∼110 fs following absorption of a single UV (400 nm = 3.1 eV) photon (Fig. 2). Our calculations (Table 1) reveal that in all cases, photoexcitation involves a transition from the O atoms to the Ni atoms in a LMCT. The electron density on the Ni atoms increases through photoexcitation and therefore exhibits a fast relaxation that is attributed to e–e scattering, similar to bulk NiO. The slightly longer lifetime of NiO over larger stoichiometric clusters is due to a lower density of states and degrees of freedom. The fraction of the excited state population (δ) that enters a long-lived (>2.5 ps) state, determined by the amplitude ratio of the fitting coefficients between the plateau function and total transient signal, reveals a decrease in the long-lived component with size of stoichiometric cluster (Table 1). A long-lived state is accessible in NiO (δ = 18%) and (NiO)2 (δ = 2.0%) but is not in larger clusters. The decreasing plateau component of the transient signal suggests that the larger clusters contain sufficiently high density of states to enable charge recombination. The potential energy curves of NiO reveal a rich density of excited states below the X3Σ− → 3Σ− photoexcitation at 3.04 eV.27 Photoelectron spectroscopy (PES) of NiO shows several low-lying excited states below 3 eV,28–31 and a ∼0.5 eV energy gap between the ground and the first excited state.27,32 Our measurements show the 3Σ− state of NiO has a lifetime of 214 ± 12 fs and may cross into a long-lived 3Σ+ or 3Δ state, which is represented by the plateau function. This is in agreement with literature results, where lower lying excited states exhibit microsecond lifetimes following visible excitation.33 Perhaps unsurprisingly, the stoichiometric clusters exhibit the fastest lifetimes due to the LMCT placing more electron density on the Ni-3d orbitals which relax through e–e scattering. These results echo the rapid relaxation dynamics reported for chromium oxide clusters, where the LMCT character of the excited state is proportional to the 10s of fs relaxation through e–e scattering amongst the d electrons.18 The e–e scattering mechanism is likely prominent in all open d-shell transition metal oxides, especially when LMCT occurs.
Cluster | τ (fs) | δ (%) | Ni-3d | Ni-4p | Ni-4s | O-2p |
---|---|---|---|---|---|---|
NiO | 214 ± 12 | 18 | 4 | 3.1 | 0 | −7 |
(NiO)2 | 128 ± 8 | 2.0 | −62 | 15 | 66 | −29 |
(NiO)3 | 111 ± 9 | 0.0 | 15 | −6 | 28 | −38 |
(NiO)4 | 107 ± 11 | 0.0 | 24 | −6 | −3 | −21 |
(NiO)5 | 106 ± 17 | 0.0 | 39 | −5 | 42 | −77 |
Oxygen deficient clusters possess longer sub-ps lifetimes (by up to 80%) than the stoichiometric series (Table 2). Such suboxides have large proportions of 3d → 4p photoexcitation which may be responsible for the long-lived states. The Ni2Ox (x < 3) series reveals gradual changes in excited state transients with O content (Fig. 3).
Cluster | τ (fs) | δ (%) | Ni-3d | Ni-4p | Ni-4s | O-2p |
---|---|---|---|---|---|---|
a Indicates growth lifetime instead of decay. | ||||||
Ni2 | 28 ± 9a | 100 | −97 | 97 | 0 | — |
Ni2O | 204 ± 18 | 32 | −95 | 32 | 55 | 9 |
Ni3 | 170 ± 51a | 100 | −92 | 92 | 0 | — |
Ni3O | 191 ± 18 | 18 | −57 | 25 | 28 | 3 |
Ni3O2 | 202 ± 23 | 13 | −17 | 3 | −6 | 20 |
Ni4O2 | 187 ± 22 | 4.0 | −65 | 19 | 39 | 3 |
Ni4O3 | 176 ± 20 | 0.0 | −71 | 24 | 35 | 9 |
Ni5O3 | 117 ± 20 | 0.0 | −71 | 17 | 42 | 4 |
Ni5O4 | 151 ± 20 | 0.0 | −54 | 4 | 43 | −15 |
Ni6O4 | 225 ± 40 | 0.0 | −59 | 5 | 39 | −14 |
Ni6O5 | 173 ± 23 | 0.0 | −13 | −2 | 6 | 10 |
Ni7O5 | 167 ± 36 | 0.0 | −21 | 0 | 15 | −8 |
Ni7O6 | 142 ± 23 | 0.0 | −20 | 1 | 7 | 5 |
Fig. 3 Transient signals for Ni2Ox (x < 3) clusters with total fit and lifetime shown. Dashed lines show the initial signal intensity. The structures and densities are similar to Fig. 2, except Ni2 is shown at an isodensity of 0.002 Å−3. |
The transient signal for Ni2 requires a growth function (τg) and shows the entire population reaches a long-lived (>2.5 ps) state. The measured τg (28 ± 9 fs) matches the cross correlation of the laser beams, suggesting the signal arises from the direct ionization of Ni2 that is present in the neutral cluster population. The ground state of Ni2 has been investigated with controversial results that arise from the interaction of two 3d94s1 Ni atoms, forming a single bond between the 4s orbitals with little 3d involvement. The electronically excited states are difficult to assign, but appear at wavelengths <450 nm.34 The UV-vis spectra of Ni2 shows several absorption bands at ∼3 eV.35,36 The dσg → pπu (3πu or C state) is strongly bound, but overlaps with the dissociative sσg → sσu (3Σu− or B state).35,37 A dδg → pπu transition (3Φu state) may also exist nearby but is expected to have weak signal intensity. No decrease in the transient Ni2+ signal is recorded, suggesting the C 3πu state of Ni2 is accessed by the 3.1 eV pump photon and is long lived due to poor overlap with lower lying states. Photoexcitation of Ni2 is strictly Ni-3d → Ni-4p in our calculations (Fig. 4). Thus, an electron is promoted from an essentially nonbonding 3d-type molecular orbital into a 4p πu orbital, with some bonding character resulting in a stronger bond.38
The density of states of Ni2O2 and Ni2O (Fig. 4) are in agreement with previous calculations.39,40 Ni2O+ is a particularly stable cation.25,41 The transient signal for Ni2O contains a slower (204 ± 18 fs) relaxation, extended by ∼60% over Ni2O2 which contains a fast (∼128 fs) decay from high LMCT. A large proportion (δ = 32%) of the photoexcited Ni2O population reaches a long-lived state which survives over 2.5 ps, proportional to the amount of d → p transition character. Strong hybridization of Ni-3d and O-2p orbitals enables photoexcitation of Ni2O to be characterized as Ni-3d → 55% Ni-4s and 32% Ni-4p. The 4s orbitals have favorable exchange interactions with the Ni-3d electrons due to large spatial overlap and are therefore strongly coupled and enable rapid relaxation through scattering, although slightly slower than the LMCT dynamics of the stoichiometric clusters.
The Ni3Ox (x < 4) clusters behave similar to the Ni2Ox clusters in that the sub-ps transient lifetime and long-lived (>2.5 ps) excited state population increases with decreased O character (Fig. 5). Ni3Ox clusters also contain a large density of states, with no obvious energy gaps (Fig. S2, ESI†). The stoichiometric cluster, Ni3O3, has the shortest lifetime of the series (111 ± 9 fs) and does not contain a long-lived plateau. With one less O atom, the lifetime of Ni3O2 (202 ± 23 fs) increases by ∼80% over the stoichiometric cluster. The transient signals for the photoexcitation of Ni3O2 and Ni3O exhibit comparable lifetimes and δ coefficients, suggesting similar excited state landscapes. Our calculations reveal that photoexcitation of both clusters contain significant Ni-3d to Ni-4s and Ni-4p transitions. The 4s component couples to the 3d electrons for a rapid relaxation. Further, the 4s character is larger than the 4p component, reflected by long-lived states that is only 13 and 18% of the total population of the Ni3O2 and Ni3O clusters, respectively.
Fig. 5 Transient signal of Ni3Ox (x < 4) clusters and their structures, similar to Fig. 2. |
The excited states of Ni3 are similar to Ni2,35,37 with dπg → pπu (bound) and the slightly higher energy sσu → sσg (dissociative) transitions near 400 nm. However, in contrast to Ni2+, the Ni3+ signal in our experiment does not appear with the laser pulse but instead exhibits a delayed growth. This delayed transient response is a strong signature of ionization that competes with photodissociation. Several possibilities might account for the growth recorded in the Ni3+ signal (τg = 170 ± 51 fs) as it matches closely with the decay lifetime for most clusters, including Ni3O (∼191 ± 18 fs). Our calculations predict a bond dissociation energy of 4.2 eV for Ni3O, in agreement with literature values,42 suggesting that it is stable and does not release O to form Ni3. Photodissociation of Ni3O2 to form Ni3 is unlikely, requiring removal of two O atoms. However, Ni3+ is not observed from the photofragmentation of oxygen rich nickel oxide cation clusters25 and Ni3+ has a small fragmentation energy.43 Thus, it is likely that Ni3 dissociates into Ni2 + Ni from the combined energy of both laser beams at temporal overlap, through the dissociative sσu → sσg excitation channel. This intermediate neutral state crosses into the stable dπg → pπu channel in ∼170 fs where it remains for the duration of the experiment (2.5 ps). Ionization then proceeds through the removal of the excited nonbonding d electron, resulting in a stable cation.
Clusters containing either 4 or 5 Ni atoms continue to follow the described trend, where the stoichiometric clusters exhibit the fastest relaxation (τ ∼ 107 fs) through LMCT, and inclusion of O vacancies enable longer lifetimes. With one less O atom from Ni4O4, the lifetime increases by 65% (Fig. 6). The photoexcitation of Ni4O3 and Ni4O2 contain similar Ni-d → Ni-s and Ni-d → Ni-p character, facilitating similar extended excited state lifetimes (τ ∼ 180 fs). Similarly, Ni5O3 and Ni5O4 contain similar dynamics, with lifetimes slightly longer than Ni5O5. The charge transfer of these clusters shows similar effects, where the Ni5O3 and Ni5O4 clusters are strongly Ni-d → Ni-s. Thus, in the 3D geometry of such larger clusters, the Ni-4p character of the excited electron becomes negligible, and no long-lived signal is obtained, demonstrating an efficient relaxation (Fig. S3, ESI†).
Fig. 6 Transient signal of Ni4Ox (x < 4) clusters and their structures, similar to Fig. 2. |
Stoichiometric clusters are not recorded with significant intensity above (NiO)5, however, the similar lifetime values and dynamics measured for a few additional suboxide clusters implies that the trend extends into larger clusters as well. The d → p transition is also not prominent in larger clusters, which instead show mainly a transition of Ni-3d → Ni-4s character. The excited state orbital characters and change in electron transition densities for Ni6O4−5 and Ni7O5−6 are presented in Fig. S4 (ESI†). Ni6O4 has an unusually long lifetime compared to the other clusters (225 ± 40 fs). This extended lifetime may be related to its uniquely high C3 symmetry (also suggested to be Td23), which may facilitate the delocalization of charge carriers for a prolonged lifetime. Photoexcitation of Ni6O4 is unique in that it contains significant Ni-3d → O-s character (up to 0.29), which is not present in other cluster excitations. The Ni7O5 and Ni7O6 clusters have lifetimes ∼29–52% greater than the expected stoichiometric lifetime of 110 fs, consistent with smaller cluster species. The transient signals for these larger clusters are presented in Fig. S5 (ESI†).
Footnote |
† Electronic supplementary information (ESI) available: Fig. 1 shows the experimental apparatus, Fig. S2–S4 shows the calculated excited state orbital contributions for nickel oxide clusters. Fig. S5 shows the shows the lifetime measurements for several larger clusters. The XYZ coordinates of all clusters are also presented. Fig. S6 presents the cluster structures and spin states. See DOI: 10.1039/d2cp00209d |
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