Electronic and Charge-Transport Properties of the
Au
3
(CH
3
NCOCH
3
)
3
Crystal: A Density Functional Theory Study
Lingyun Zhu,
†,§
Veaceslav Coropceanu,
†
Yuanping Yi,
†,∥
Bhaskar Chilukuri,
‡
Thomas R. Cundari,
‡
and Jean-Luc Bre ́ das*
,†,⊥
†
School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta,
Georgia 30332-0400, United States
‡
Department of Chemistry Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton,
Texas 76203, United States
* S Supporting Information
ABSTRACT: Density functional theory was used to inves-
tigate the electronic and charge-transport properties of the
trinuclear gold Au
3
(CH
3
NCOCH
3
)
3
crystal. Hole transport
is found to be anisotropic and characterized by a very small
effective mass of about 0.21 m
0
along the stacking direction of
the Au
3
molecules. Interestingly, the calculations suggest an
isotropic character of electron transport, for which the effective
mass is about 1 m
0
. We show that while the interstack
interactions facilitate electron transport in the directions perpendicular to the stacks, they act to diminish this transport along the
stacking directions. Overall, the present results indicate that this compound is a promising ambipolar material for application in
electronic devices.
SECTION: Energy Conversion and Storage; Energy and Charge Transport
G
old-trimer based systems have recently received signifi-
cant attention due to their interesting properties
1−3
related to π-acidity and basicity, luminescence, thermochrom-
ism, or solvoluminescence, and their potential for applications
in metal−organic electronic devices.
4,5
For instance, upon
irradiation with UV light, tris((μ
2
-methylimino(methoxy)-
methyl)-gold(I)) (Au
3
(CH
3
NCOCH
3
)
3
) exhibits a long-
lived yellow emission with a lifetime of about 31 s; in addition,
when a previously photoirradiated crystal Au
3
(CH
3
N
COCH
3
)
3
is dropped into a good solvent, a bright burst of
yellow light detectable by the human eye is produced
(solvoluminescence).
6−8
In general, Au
3
(CH
3
NCOCH
3
)
3
can crystallize into three polymorphic forms: hexagonal,
triclinic, and monoclinic.
7
The hexagonal polymorph is the
only one that displays solvoluminescence and in which the
gold(I) ions of Au
3
(CH
3
NCOCH
3
)
3
form extended chains.
It was suggested that solvoluminescence involves energy
storage that is facilitated by charge-carrier mobility along the
gold chains.
8
Despite significant interest in this system, only
limited theoretical studies have been reported to date.
9,10
Here, we use quantum mechanical methods to study the
electronic and charge-transfer properties of the hexagonal form
of the Au
3
(CH
3
NCOCH
3
)
3
crystal. To the best of our
knowledge, our work represents the first study of the charge-
transport parameters in this class of materials.
The electronic-structure calculations on the crystal were
performed at the density functional theory (DFT) level using
the Perdew−Burke−Ernzerhof (PBE) exchange-correlation
functional with a plane-wave basis set (300 eV cutoff) and
projector augmented wave (PAW) potentials.
11,12
The self-
consistent calculations were carried out with 4 × 4 × 14 and 4
× 4 × 8 k-point meshes for structures based on the original unit
cell and a doubled unit cell, respectively. The inverse effective
mass tensor for the three-dimensional crystal, m
ji
−1
, is defined as
=
ℏ
∂
∂ ∂ m
E
k k
1 1
ij j i
2
2
(1)
where subscripts i and j denote the Cartesian coordinates in
reciprocal space, E is the band energy, ℏ is the Planck constant,
and k is the electron wavevector. The inverse effective mass
tensor was calculated assuming dk = 0.01/Bohr. All the DFT
crystal-structure calculations were carried out using the VASP
5.2 code.
13
The transfer integrals (electronic couplings) were evaluated
by using a fragment orbital approach in combination with a
basis set orthogonalization procedure.
14
Since the frontier
valence levels of Au
3
(CH
3
NCOCH
3
)
3
are 2-fold degenerate
(labeled here as H and H-1), the electronic couplings for holes
are evaluated by considering effective transfer integrals defined
as
= + + +
‐ ‐ ‐ ‐
t t t t t [( )/2]
h
eff
H,H
2
H 1,H
2
H,H 1
2
H 1,H 1
2 1/2
(2)
Received: May 6, 2013
Accepted: June 18, 2013
Published: June 18, 2013
Letter
pubs.acs.org/JPCL
© 2013 American Chemical Society 2186 dx.doi.org/10.1021/jz400950v | J. Phys. Chem. Lett. 2013, 4, 2186−2189