NANOMATERIALS Three-orders-of-magnitude variation of carrier lifetimes … · NANOMATERIALS Three-orders-of-magnitude variation of carrier lifetimes with crystal phase of gold nanoclusters

NANOMATERIALS

Three-orders-of-magnitude variationof carrier lifetimes with crystal phaseof gold nanoclustersMeng Zhou1*, Tatsuya Higaki1*, Guoxiang Hu2, Matthew Y. Sfeir3†‡, Yuxiang Chen1,De-en Jiang2§, Rongchao Jin1§

We report a three-orders-of-magnitude variation of carrier lifetimes in exotic crystallinephases of gold nanoclusters (NCs) in addition to the well-known face-centered cubicstructure, including hexagonal close-packed (hcp) Au30 and body-centered cubic (bcc)Au38 NCs protected by the same type of capping ligand. The bcc Au38 NC had anexceptionally long carrier lifetime (4.7 microseconds) comparable to that of bulk silicon,whereas the hcp Au30 NC had a very short lifetime (1 nanosecond). Although the presenceof ligands may, in general, affect carrier lifetimes, experimental and theoretical resultsshowed that the drastically different recombination lifetimes originate in the differentoverlaps of wave functions between the tetrahedral Au4 building blocks in the hierarchicalstructures of these NCs.

Light-harvesting nanomaterials in solar en-ergy utilization (1, 2) convert absorbed lightinto excitons (electron–hole pairs). Excitonscan dissociate productively to form freecharge carriers or recombine unproductively,

so the relative rates of these processes are im-portant in energy storage and conversion. Cor-relating the carrier recombination dynamics andthe structure of nanomaterials is of great impor-tance. Carrier lifetimes are dependent on theband gap energy (Eg), overlap between the wavefunctions of the ground state and the excitedstate, temperature, and other conditions (3, 4).Manipulation of carrier lifetimes can greatlyalter the functionalities of nanomaterials fordifferent applications.Metal nanoclusters (NCs) hold promise in a

variety of applications (5–8) owing to theirversatile functionalities that can be tailored bysize, structure, and composition (9, 10). Unlikeplasmonic gold nanoparticles (Au NPs), ultra-small Au NCs (<2 nm in diameter) show discreteelectronic energy levels and multiple peaks intheir ultraviolet-visible (UV-vis) absorption spec-tra (11). Achieving a fundamental understandingof the optical properties and photophysics ofmetal NCs (including the electron and phonondynamics) is of great importance to the explo-ration of their applications (12, 13). Ultrafastspectroscopy has revealed that Au NCs typicallyshow fluence-independent electron dynamics

(14, 15), which is different from the behavior ofplasmonic Au NPs (16, 17) and semiconductorquantum dots (18, 19).With respect to the effect of NC size on photo-

physics, a general trend is that the larger NCshave shorter carrier lifetimes because of a smallerEg. Such an energy gap trend was recently re-ported in Au NCs (20). Although excited-state

lifetimes generally follow the Eg law, AuNCswithEg > 1 eVmay showdeviations (21). Therefore, thestructure of NCs should play an important role intheir carrier lifetimes because the quantum con-finement of electrons is dictated by the shape ofthe potential well.Bulk gold adopts the face-centered cubic (fcc)

structure, but Au NCs can adopt many differenttypes of structures (22–24), including hexagonalclose-packed (hcp) (25) and body-centered cubic(bcc) (26) structures. The different packing ofAu atoms gives rise to different electronic struc-tures andUV-vis absorption. The fcc series (Au28,Au36, Au44, and Au52, which are protected by thesame thiolate ligand) adopted a layer-by-layergrowth pattern and thus showed a uniformevolution in UV-vis absorption (21, 24). Uponphotoexcitation in the fcc NCs, electron coolingoccurred only in the metal core and there wasno core–shell charge transfer (21), unlike similarlysized icosahedral NCs (27). Recently, excited-stateelectron localization was observed in the lineartriicosahedral Au37 NC largely due to its aniso-tropic shape (28).These reported examples pertain to the fcc

(21) or icosahedral (13, 28) NCs. Here, we reportunusual carrier dynamics of Au NCs with hcpand bcc crystalline phases. Specifically, the carrierdynamics of hcp Au30(S-Adm)18 (hereafter Au30,where “S-Adm” stands for 1-adamantanethiolate)and bcc Au38S2(S-Adm)20 (hereafter Au38) NCsexhibit drastic differences compared with theicosahedral Au25 and fcc Au36/Au44/Au52 NCs,

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1Department of Chemistry, Carnegie Mellon University,Pittsburgh, PA 15213, USA. 2Department of Chemistry,University of California, Riverside, CA 92521, USA. 3Centerfor Functional Nanomaterials, Brookhaven NationalLaboratory, Upton, NY 11973, USA.*These authors contributed equally to this work. †Present address:Photonics Initiative, Advanced Science Research Center, CityUniversity of New York, New York, NY 10031, USA. ‡Presentaddress: Department of Physics, Graduate Center, City Universityof New York, New York, NY 10016, USA.§Corresponding author. Email: [email protected](R.J.); [email protected] (D.J.)

Fig. 1. X-ray structures and steady-state UV-vis absorption spectra of Au30(S-Adm)18 andAu38S2(S-Adm)20 NCs. (A) Core–shell structure of Au30. (B) Core–shell structure of Au38.(C and D) UV-vis absorption spectra of Au30(S-Adm)18 and Au38S2(S-Adm)20. The arrows indicatetheir lowest-energy absorption bands. The inset in (C) shows an Au18 kernel in four layers of an hcpstructure, and the inset in (D) shows an Au30 kernel in a bcc structure with the two bcc unit cellshighlighted in red and green. Color labels: yellow, S atoms; gray, C atoms. H atoms are omitted forclarity; all other colors are for Au.

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although all six NCs possess comparable bandgaps (1.3 to 1.77 eV). Surprisingly, hcp Au30 hada substantially shorter lifetime (1 ns) than thetypical lifetime (~100 ns) of NCs with similarEg values, whereas the bcc Au38 had a substan-tially longer lifetime (4.7 ms). We argue that the~4700 times difference in lifetime between thehcp Au30 and bcc Au38 originated from the dif-ferent arrangements of local Au4 motifs withinthe hcp and bcc cores.The syntheses of hcp Au30 and bcc Au38 NCs

followed our previous methods (25, 26), andtheir structures are shown in Fig. 1, A and B.The hcp Au30 comprises an Au18 kernel in whichfour layers of atoms (Au3/Au6/Au6/Au3) are ar-ranged in the a/b/a/b manner (inset of Fig. 1C).The Au18 kernel is protected by six dimericstaples (-S-Au-S-Au-S-). The bcc Au38 comprisesan Au30 kernel in which bcc unit cells can beseen clearly (inset of Fig. 1D) and the kernel isprotected by four dimeric staples, two sulfidosand eight bridging thiolate (-S-).The steady-state absorption spectrum of hcp

Au30 shows prominent absorption peaks at 370and 545 nm, as well as a hump at 480 nm anda shoulder at 680 nm (Fig. 1C), whereas bcc Au38shows prominent peaks at 640 and 740 nm anda hump at 580 nm (Fig. 1D). By extrapolatingabsorbance to 0, the energy gaps of hcp Au30and bcc Au38 were determined to be 1.55 and1.45 eV, respectively (fig. S1).To further understand the role of core and

surface in the excited states, we compared thetransient absorption (TA) spectra pumped at360 nm and probed between 430 and 810 nmfor both NCs (Fig. 2, A and B). By the time (t) of10 ps after the pump, hot carriers in both sam-ples had cooled down and three ground-statebleaching (GSB) peaks were seen in both cases.In hcp Au30, these peaks were at 480, 545, and680 nm, together with excited-state absorption(ESA) at 600 nm (Fig. 2A). The 680-nm GSBobserved in Au30 corresponds to the shoulderband at 680 nm in the UV-vis absorption spec-trum (Fig. 1C). In bcc Au38, GSB peaks were at580, 640, and 760 nm, which were overlappedwith ESA at 500 and 700 nm (Fig. 2B). The TAspectra for both NCs showed somewhat similarprofiles, with TA in Au38 redshifted comparedwith that of Au30. There was no real ESA peakin the TA spectra; all of the ESA peak positionsagreed with those minima in the steady-stateabsorption spectra. The very broad ESA spannedthe entire visible region and even into the near-infrared, and this broad ESA overlapped withmultiple GSB peaks. Such features have beenwidely observed in other thiolate-protected AuNCs (21). The broad ESA is helpful in solar celland photocatalysis applications that require con-tinuous white light excitation so that reexcita-tion of excited states helps to maintain a longerexcited-state lifetime.Despite similar TA profiles at t = 10 ps for the

two NCs, drastic differences in carrier dynamicswere observed. In Au30, after photoexcitation at360 nm, the broad ESA disappeared within 5 ps(Fig. 2, C and D), which we attribute to hot-carrier

relaxation. In the subsequent 2 ns, most of theTA signal disappeared. Global fitting requiredthree decay components to fit the dynamics (1.2 ps,4 ps, 1 ns, fig. S2A). For excitation at 560 nm,the fast decay component was accelerated to0.8 ps, but the slow components (3.4 ps and1.1 ns) remained the same (figs. S2B and S3).The 1-ns lifetime in Au30 is very short consid-ering its Eg of 1.55 eV; for comparison, the Eg ofthe Au25 NC is 1.3 eV (11) and its excited-state

lifetime is ~100 ns, which is a typical value ofthiolate-protected Au NCs (29). A recent studyalso reported a relatively short lifetime (~3 ns)in Au30(SR)18 (where R indicates t-butyl group)NCs with a different structure and ligand (30).In bcc Au38, the broad ESA for excitation at

360 nm decayed within < 2 ps, giving rise to anet negative GSB at 640 and 760 nm (Fig. 2, Eand F). In contrast to that in Au30, the TA sig-nal showed almost no decay between 10 ps and


Fig. 2. Comparison of spectral features and carrier dynamics of the two NCs. (A and B) TAspectra (black) of (A) hcp Au30 and (B) bcc Au38 NCs at a time delay of 10 ps pumped at 360 nm.Steady-state absorption spectra (gray) are also shown for comparison. (C and D) TA data mapwith excitation of 360 nm and kinetic traces of the hcp Au30 NC. (E and F) TA data map withexcitation of 360 nm and kinetic traces of the bcc Au38 NC. (G) Data map of ns-TA in bcc Au38 NCsbetween 0.01 and 20 µs with an excitation of 480 nm. (H) Kinetic traces probed at 620 nm andthe corresponding fit. DA, change in absorbance; mOD, milli–optical density units.

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2 ns (Fig. 2F), which suggests that it has a sub-stantially longer excited-state lifetime than thatof Au30. To obtain the complete excited-statelifetime of bcc Au38, a nanosecond TA measure-ment with excitation at 490 nm was performed(Fig. 2G), which gave a single exponential decaylifetime of 4.7 ms (Fig. 2H). With the input of the4.7 ms into the femtosecond dynamics, globalfitting showed three decay components, 0.6 ps,4.8 ps, and 4.7 ms (fig. S4A), for fitting the re-laxation dynamics. With excitation at 730 nm, thefast-decay component disappeared and only the5-ps and 4.7-ms components remained (figs. S4Band S5). The ~4700 times difference in excited-state lifetimes of the hcp Au30 and bcc Au38 NCswas unexpected given their similar Eg (~1.5 eV)according to the energy gap law.The excited-state lifetimes of Au30 and Au38

NCs can be further compared with those of

other NCs of similar Eg (Fig. 3, A to D), includingthe icosahedral Au25(SR)18 and fcc Au52(SR)28,Au44(SR)32, and Au36(SR)24 NCs with Eg between1.3 and 1.77 eV. The excited-state lifetimes of thelatter four NCs were all reported to be ~100 nsand did not follow the energy gap law (21). Thesedifferences in carrier lifetimes could arise fromthe differences in molecular orbital distribution,core–shell interactions, and metal core struc-tures. Surface ligands and the overlap of thesulfur orbitals with the Au electronic structurecan affect the carrier dynamics (31), but the bccAu38 and hcp Au30 have the same type of lig-ands. Moreover, the metal cores of both NCswere protected by dimeric staples (-S-Au-S-Au-S-), which rules out surface differences instaple type.A core–shell relaxation model was previously

proposed to explain the picosecond relaxation

in the Au25 NC (27, 31). In hcp Au30 and bcc Au38,the picosecond decay was always observed in-dependently of the pump wavelength (figs. S2to S5). The 4- to 5-ps process in both NCs couldbe explained as core–shell charge transfer orenergy relaxation within the metal core. In hcpAu30, the amplitude of the picosecond compo-nent was larger than that of the bcc Au38 (figs. S2and S4), which indicates the stronger picoseconddecay in hcp Au30.For both hcp Au30 and bcc Au38 NCs, careful

analyses of Au–Au bond-length distributionsrevealed that their cores can indeed be viewedas several locally segregated Au4 tetrahedralunits (i.e., very short bond lengths within eachAu4 versus longer distances between Au4 units,figs. S6 and S7). In the hcp Au30, the Au18 coreconsisted of six Au4 units (Fig. 3A) assembledby sharing two vertexes of each Au4, so the dis-tance between Au4 units was zero (Fig. 3E).Such a conjugated arrangement of Au4 unitsleads to a large overlap of the wave functions ofAu4 units. However, the bcc Au38 had four Au4units in the core and the distance between Au4units was ~2.86 Å (Fig. 3B and fig. S7, II). Thenonconjugated Au4 units and the longer dis-tance between them led to much less overlapof the wave functions of Au4 units and slowerenergy dissipation from the excited state to theground state.In the fcc series of NCs (Au36, Au44, and Au52),

the distances between neighboring Au4 unitswas ~3.0 Å (Fig. 3C and fig. S8) and energydissipation would be slow, but the Au4 units inthe fcc series were arranged in a double-helixpattern (24, 32); that is, Au4 units shared onevertex between neighboring units within eachchain (Fig. 3C). Theoretical analysis (33) alsorevealed 1s-like superatomic orbitals of Au4 units:4-centered-2-electron bonds (hereafter 4c-2e).Such an arrangement should lead to relativelymore efficient energy transfer within each helix.Therefore, a moderate carrier lifetime (~100 ns)is observed lying between the very short life-time of the compact, ring-like, conjugated Au4superstructure in the hcp Au30 (Fig. 3A) andthe very long lifetime of the loose, square-like,nonconjugated Au4 network in the bcc Au38(Fig. 3B).This relation between the carrier lifetime and

the Au4 network was further corroborated by theanalysis of the frontier orbitals (34). As shown inFig. 3F, the conjugated Au4 network in the hcpAu30 led to an almost zero distance between itshighest occupied and lowest unoccupied molec-ular orbital (HOMO and LUMO, respectively)and the shortest carrier lifetime. By contrast,the nonconjugated Au4 network in the bcc Au38showed the largest geometric separation ofHOMOand LUMO centroids among the three NCs andthus took the longest time for the excited stateto relax back to the ground state. Overall, thespecific arrangements of Au4 units in the NCsexplained the drastically different excited-statelifetimes resulting from the different extents ofoverlap of 4c-2e bonds. Recent work reportedthat the Au–Au distance in the metal core could


Fig. 3. Correlation between structures and excited-state lifetimes of bcc, hcp, and fcc NCs.(A to C) Tetrahedral Au4 networks in Au30, Au38, and Au36 NCs. (D) Excited-state lifetimesversus Eg of several gold NCs. (E) Excited-state lifetimes versus distance between the Au4 unitsin the cores of bcc Au38, hcp Au30, and fcc Au36/Au44/Au52 NCs. (F) Frontier orbitals andHOMO-LUMO centroid distances of Au30, Au38, and Au36 from DFT calculations. Color labels:yellow, S atoms; all other colors indicate Au. Carbon tails are omitted for clarity.



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be affected by surface functional group (35, 36).To understand how the functional group affectsthe core structure, we performed density func-tional theory (DFT) calculations and optimizedthe structure of the hcp Au30 NC with R=Admin comparison with R=CH3. The ligand effecton the Au–Au distances is rather minor in thiscase (fig. S9). The simulated optical absorptionspectra of hcp Au30 and bcc Au38 (see supple-mentary text and fig. S10) show good agreementwith the experiment (fig. S1), further indicatingthat the optical gaps and oscillator strengths arenot key factors in dictating their excited-statelifetimes.A closer examination of the early part of the

TA dynamics also revealed different behaviorfor hcp Au30 and bcc Au38 (Fig. 4, A to F). Strongoscillation behavior was observed in the hcpAu30 TA decay traces between 510 and 560 nm(Fig. 4, A and D), which originate from coherentphonons. Coherent phonons were previouslyobserved in Au NCs (13, 28, 37) and were causedby ultrafast photoexcitation. The frequency ofthe phonons in hcp Au30 was determined to be16.7 cm−1 by fast Fourier transform (fig. S11).The oscillation only persisted for two periodsand was totally damped in <4 ps. The fast damp-ing of oscillation suggested that the energy lossin Au30 was very fast, which agreed with its rapidexcited-state relaxation. By contrast, no oscilla-tory feature was observed in the TA time profileof bcc Au38 (Fig. 4, B and E). The 16.7 cm−1

phonon frequency of hcp Au30 was ascribed tothe acoustic phonon in themetal core. The giantAu246(SR)80 NC (15) had a similar phonon fre-quency of ~16.7 cm−1 (Fig. 4, C and F), whereasAu25, which has a size similar to that of hcp Au30,

exhibited coherent phonons with much higherfrequencies (40 and 80 cm−1) (27). In plasmonicAu NPs, the frequency of the coherent vibrationwas inversely proportional to the particle diam-eter (16), but in ultrasmall Au NCs, the aboveresults indicate that the structure rather thanthe size plays an important role in the phononfrequency.We have demonstrated a three-orders-of-

magnitude variation of carrier dynamics withcrystalline phases of hcp Au30 and bcc Au38 NCsthat relates to the distance between the Au4tetrahedral units and their connection modes.The extraordinarily long lifetime of 4.7 ms inbcc Au38 is comparable to that of bulk siliconand is much longer than that of semiconductorquantum dots, so this NC material may holdpromise in boosting the NC solar-cell perform-ance (8, 38). The correlation of the structureand photodynamics of these metal NCs may stim-ulate their future applications in solar energy con-version, photocatalysis, and other optoelectronicprocesses.

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ACKNOWLEDGMENTS

Funding: R.J. acknowledges financial support from the NationalScience Foundation (DMR-1808675) and the Air Force Office ofScientific Research. D.J. was supported by the U.S. Department ofEnergy, Office of Science, Office of Basic Energy Sciences,Chemical Sciences, Geosciences, and Biosciences Division. Thiswork used resources of the Center of Functional Nanomaterials,which is a U.S. DOE Office of Science Facility at BrookhavenNational Laboratory under contract no. DR-SC0012704.Author contributions: M.Z. and M.Y.S. performed all TAmeasurements and M.Z. performed the data analysis.T.H. prepared Au30 and Au38 NCs and Y.C. performed someof the steady-state measurements. G.H. and D.J. performed theDFT calculations and analysis. R.J. designed the study andsupervised the project. M.Z. and R.J. wrote the manuscript withcontributions from all authors. Competing interests: Theauthors declare no competing interests. Data and materialsavailability: All data are available in the manuscript and in thesupplementary materials. All data needed to evaluate ourconclusions are provided in the manuscript or in thesupplementary materials.

SUPPLEMENTARY MATERIALS

science.sciencemag.org/content/364/6437/279/suppl/DC1Materials and MethodsSupplementary TextFigs. S1 to S11References (39–43)

25 January 2019; accepted 22 March 201910.1126/science.aaw8007


Fig. 4. Oscillations observed in NCs. (A to C) TA data map of hcp Au30, bcc Au38 (pumped at360 nm), and Au246 (pumped at 470 nm) NCs between –1 and 16 ps. (D to F) Kinetic traces probedat selected wavelengths. Strong oscillations were observed in Au30 and Au246 but not in Au38.



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nanoclustersThree-orders-of-magnitude variation of carrier lifetimes with crystal phase of gold

Meng Zhou, Tatsuya Higaki, Guoxiang Hu, Matthew Y. Sfeir, Yuxiang Chen, De-en Jiang and Rongchao Jin

DOI: 10.1126/science.aaw8007 (6437), 279-282.364Science

, this issue p. 279Science5 microseconds, which is comparable to bulk silicon.∼lifetime of

cluster had a381 nanosecond), and a body-centered cubic Au∼ cluster had a much shorter lifetime (30close-packed Augreatly affect carrier lifetimes. Despite having similar bandgaps to those of face-centered cubic clusters, a hexagonal

found that atomic packing and molecular orbital overlap canet al.100 nanoseconds. Zhou ∼lifetime of these excitons is ) that adopt the usual face-centered cubic packing, the40 to Au30ligand-capped gold clusters of 30 to 40 atoms (Au

Like semiconductors, small metallic clusters can absorb light and create excitons (electron-hole pairs). InAtomic packing controls exciton lifetime

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