Low-Temperature Magnetism in Nanoscale Gold Revealed through

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Spectroscopy and Photochemistry; General Theory

Low Temperature Magnetism in Nanoscale Gold Revealed Through Variable-Temperature Magnetic Circular Dichroism Spectroscopy Patrick J. Herbert, Phillip Window, Christopher J. Ackerson, and Kenneth L. Knappenberger J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03473 • Publication Date (Web): 24 Dec 2018 Downloaded from http://pubs.acs.org on December 25, 2018

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The Journal of Physical Chemistry Letters

Low Temperature Magnetism in Nanoscale Gold Revealed Through Variable-Temperature Magnetic Circular Dichroism Spectroscopy Patrick J. Herbert1, Phillip Window,2 Christopher J. Ackerson,2 Kenneth L. Knappenberger, Jr.1*

1

Department of Chemistry, The Pennsylvania State University, University Park,

PA 16802

2

Department of Chemistry, Colorado State University, Fort Collins, CO, 80523

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT:

The

low-temperature

(0.35

-

4.2

K)

Page 2 of 26

steady-state

electronic

absorption of the monolayer-protected cluster (MPC) Au102(pMBA)44 was studied using magnetic circular dichroism spectroscopy to investigate previously reported low-temperature

( 100K) results in lattice contraction. This temperature dependence coincides with the magnitude of

magnetic

anisotropy;

lattice

expansion

results

in

increasing

magnetic

anisotropy and lattice contraction results in decreasing magnetic anisotropy. This structure-dependent magnetism behavior is interpreted as arising from changes


12

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to d-d orbital interactions due to changes in bond length between neighboring Au atoms.8,25 Spin-orbit coupling interactions in MPCs have been invoked to describe

the

electronic

structure

of

core-based

states.26

These

spin-orbit

coupling dependent electronic properties include the observation of oxidationdependent

magnetism.27,28

For

MPCs

the

Au-Au

core

lattice

surface

is

passivated with inorganic thiol staple units. As a result, the influence of these staple units on the Au-Au core lattice must be considered. Yamazoe, et. al. reported on the relative temperature dependence of Au-Au and Au-S bond stiffnesses

for

a

series

of

Aun(PET)m

MPCs,

where

PET

stands

for

phenylethane thiol.29 X-ray absorption fine structure (EXAFS) analysis indicates that the bonding structure of MPCs can be classified by Au-S and two Au-Au bonds with distinct bond lengths. Temperature-dependent analysis of each bond revealed an increase in the Debye-Waller (DW) factors for both Au-Au bonds. DW

factors

independent.

corresponding

to

Au-S

bonds

were

relatively

temperature

The DW factor correlates to the statistical distribution of the

measured bond length relative to a center lattice point.30 An increase in DW


13


Page 14 of 26

factor at low temperature suggests a small energy barrier for processes such as lattice expansion.31 For core-based Au-Au bonds, the low thermal activation barrier

to

lattice

expansion

may

explain

the

pronounced

temperature

dependence of the Landé g-factor for the Au102(pMBA)44 interband transition. Expansion of the core lattice decreases d-orbital coupling between Au-Au atoms, thereby increasing the d-band vacancy.8-10 Larger d-band vacancy results in greater paramagnetic behavior and therefore larger g-factor values.

1.6 eV component. The electronic transition at 1.6 eV is expected to arise from states localized to the -S-Au-S- staple unit motif.16-18 From temperature-dependent analysis of DW factors, Einstein temperatures corresponding to the Au-Au and Au-S bonds for a series of MPCs have been previously reported.29 The Einstein temperature correlates to the temperature at which a quantifiable change to the DW factor for a bond is observed.32 A high Einstein temperature for a given bond suggests that the bond-length is insensitive to sample temperature. For Au-S bonds in MPCs an approximate 50% increase in the Einstein temperature


14

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is reported relative to the lattice Au-Au bonds. The inelasticity of the Au-S bond is attributed to formation of a π-bonding network between the gold, sulfur, and ligand-based

carbon

atoms.14

This

Au-S

bond

inelasticity

suggests

that

structure-dependent thermal perturbation to the ground state electronic structure should be minimal over the range sampled for the VTVH-MCD measurements. The temperature dependence of the MCD response, can also result from thermal population redistribution from lower to higher ground state energy levels. This redistribution depends on the energy separation between states determined by zero-field splitting (ZFS).23 Magnetic circular photoluminescence (MCPL) spectroscopy measurements on the Au25(SC8H9)18 MPC quantified ZFS values for staple unit states on the order of several hundred μeV.33 At 4.2 K (kBT = 0.36 meV), Boltzmann population redistribution for semiring-based states is

meaningful.

The

small

temperature-dependent

increase

in

the

g-factor

quantified for semiring states may arise from thermal population redistribution. We note that the MCD feature at 1.36 eV was too low in amplitude for reliable


15


Page 16 of 26

VTVH-MCD analysis, but its inclusion was necessary to accurately fit the spectra.

We report here state-resolved magnetic anisotropy for the atomicallyprecise Au102(pMBA)44 monolayer-protected cluster. Three absorption features centered at 1.36 eV, 1.6 eV and 2.6 eV were observed in MCD differential spectra.

Previously

reported

DFT

calculations

suggest

that

the

2.6

eV

component arises from excitation of the d-band. Conversely, DFT calculations predict the 1.6 eV transition corresponds to excitation of electrons localized to semiring units. Electronic properties of structure-dependent were separately spectroscopically addressed through MCD spectroscopy. VTVH analysis of MCD signal amplitude for the 1.6 eV and 2.6 eV components revealed distinct magnetic behaviors. At 4.2 K, Landé g-factors for the 1.6 eV and 2.6 eV component were quantified as 0.15 and 1.5, respectively. For spin-orbit coupled systems, g-factors >1 correspond to unpaired electrons. The g-factor of 1.5 for the 2.6 eV component suggests that there are d-band vacancies. This agrees with other reports of low-temperature magnetism in nanogold systems.4,7 The


16

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temperature-dependent increase in the Landé g-factor for the 2.6 eV component is attributed to expansion of the Au-Au core lattice. This Au-Au bond expansion results in changes to the s-p-d orbital hybridization. Due to this rehybridization, the 5d10-δ6s1+δp electron configuration results in d-orbital vacancies (δ > 0) and magnetic behavior.

The observation of low-temperature magnetism in nanoscale noble metal systems is of both fundament and practical interest. Oxidation-dependent roomtemperature paramagnetism has been reported for MPCs.27,28,34 Furthermore, long-range lower-temperature magnetic ordering has been observed in these inherently

paramagnetic

MPC

assemblies.35,36

In

contrast

to

this

study,

oxidation-dependent magnetism in MPCs arise from unpaired electrons residing in superatomic orbitals. This chemically-induced magnetic behavior suggests MPCs could serve as functional materials for spintronic and magnetic heating applications. The results reported here indicate that the magnetic behavior of MPCs is additionally dependent on structural parameters (i.e. Au-Au bond length).

These

structural

parameters

directly

influence


spin-orbit

coupling

17


Page 18 of 26

between neighboring atoms, specifically d10-d10 spin-orbit coupling of Au atoms. Structure-dependent changes to d10-d10 spin-orbit coupling interactions can result in s-p-d orbital rehybridization, leading to Au d-band vacancies and magnetic anisotropy. Given the atomically precise nature of MPCs, these clusters can serve as a model system for understanding d-d orbital interactions and magnetic anisotropy in few Au-atom containing clusters.

EXPERIMENRTAL METHODS

The Samples

synthetic of

protocol

Au102(pMBA)44

for were

Au102(pMBA)44 is dispersed

in

a

reported

in

10%

(w/w)

literature.37 polyvinyl

alcohol/water solution. The solution was then dropcast onto a quartz slide and followed by the placement of a second quartz slide to enclose the film sample. The film sample was then allowed to dry in a vacuum desiccator. For MCD measurements, the sample was affixed to a probe and loaded into the bore of a 10 T superconducting magnet (Oxford). Sample temperatures reaching 0.35 K were attained through a He3 cooled cryostat. Variable temperatures were controlled using a resistive heater attached to the sample probe. Excitation of


18

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the sample was achieved using the output of a quartz-tungsten-halogen (QTH) 250 W lamp (Newport). The excitation center wavelength was scanned using a monochromator and focused onto the sample. The excitation photons were varied

between

left-

and

right-circular

polarization

using

a

photoelastic

modulator. Differential absorption of circularly polarized light at each scanned wavelength was detected using a Si photodiode. A lock-in amplifier was used to synchronize absorption events to specific polarization states.

AUTHOR INFORMATION The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by an award from the National Science Foundation to K.L.K., under Grant Number CHE-1806222, and a grant from the National Science Foundation (CHE-1455099) to C.J.H. REFERENCES


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Page 20 of 26

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Low-Temperature Magnetism in Nanoscale Gold Revealed through

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