Simplifying the conductance profiles of molecular junctions: The use of the trimethylsilylethynyl moiety as a molecule-gold contact

Article abstract of DOI:10.1039/c2dt31825c

Conductance across a metal|molecule|metal junction is strongly influenced by the molecule-substrate contacts, and for a given molecular structure, multiple conductance values are frequently observed and ascribed to distinct binding modes of the contact at each of the molecular termini. Conjugated molecules containing a trimethylsilylethynyl terminus, -C≡CSiMe ₃ give exclusively a single conductance value in I(s) measurements on gold substrates, the value of which is similar to that observed for the same molecular backbone with thiol and amine based contacting groups when bound to under-coordinated surface sites.

Full text of DOI:10.1039/c2dt31825c

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Simplifying the conductance profiles of molecular
junctions: the use of the trimethylsilylethynyl moiety
Cite this: Dalton Trans., 2013, 42, 338
as a molecule–gold contact†‡
Santiago Marqués-González,a Dmitry S. Yufit,a Judith A. K. Howard,a
Santiago Martín,*b,c Henrry M. Osorio,d Víctor M. García-Suárez,e Richard J. Nichols,f
Simon J. Higgins,f Pilar Ceac,d and Paul J. Low*a
Received 10th August 2012,
Accepted 9th October 2012
DOI: 10.1039/c2dt31825c
www.rsc.org/dalton
Conductance across a metal|molecule|metal junction is strongly significant number of individual measurements on a junction
influenced by the molecule–substrate contacts, and for a given typically reveal a range of conductance values arising from not
molecular structure, multiple conductance values are frequently only the number of molecules trapped within the junction,2
observed and ascribed to distinct binding modes of the contact at but also from variations in the nature of the molecule-sub-
each of the molecular termini. Conjugated molecules containing a strate contact, the tilt-angle of the molecule to the surface,3
trimethylsilylethynyl terminus, –CuCSiMe3 give exclusively
a
and the site of binding on flat terraces or step edges4 and adja-
single conductance value in I(s) measurements on gold substrates, cent neighbouring adatoms.5 The measured conductance of
the value of which is similar to that observed for the same mole- an individual molecular junction is therefore influenced by
cular backbone with thiol and amine based contacting groups both the chemical composition of the contacting group and
when bound to under-coordinated surface sites.
the structure of the local, accessible binding sites on the elec-
trode surfaces. Given the different degrees of surface rough-
ness, and hence range of accessible molecule–surface binding
sites, associated with the different measurement platforms
(e.g. break junction methods vs. the use of a pristine STM tip
in I(s) methods), the measurement method can dictate the
range of contact types observed.5 Low conductance type A con-
tacts are due to molecular binding at low coordination surface
sites, whilst the progressively more conductive contacts are
due to molecular binding at higher coordinate defect sites at
one (type B) or both (type C) contact surfaces. Type C contacts
are commonly observed in MCBJ and STM-BJ measurements,
but are generally less often observed in junctions formed
through the softer I(s) and I(t) methods.4 This electrical varia-
bility arising from the site of molecule–surface binding may
limit the use of molecules as active components within device
structures, and contacting groups that permit the assembly of
robust, reproducible and stable molecular junctions are to be
desired.6,7
Trimethylsilylethynyl has emerged as a promising contact-
ing group, able to form stable, and often well-ordered, self-
assembled monolayers on Au(111) surfaces.8 The arrangement
of molecules in the SAM is consistent with the silyl moiety
either being bound to three-fold hollow sites or a top single
metal atoms on the Au(111) surface, whilst the presence of
single atom-step deep etch-pits provides evidence for chemi-
sorption similar to thiol on gold interactions.8 The trimethylsilyl-
ethynyl terminated oligophenylene ethynylene (OPE) 1a
(Fig. 1) forms contacts to gold substrates in both Langmuir–
Single molecule electronics science has advanced rapidly
through the introduction of reliable methods for the measure-
ment of trans-molecule conductance in various metal|mole-
cule|metal configurations, such as mechanically controlled
break junction (MCBJ), STM-break-junction (STM-BJ), conduct-
ing probe AFM (CP-AFM), nano-pore devices, and STM-based
matrix isolation, I(s) and I(t) methods.1 Data from a statistically
aDepartment of Chemistry and Centre for Molecular and Nanoscale Electronics,
Durham University, Durham, DH1 3LE, UK. E-mail: p.j.low@durham.ac.uk;
Fax: +44 (0)191 384 4737; Tel: +44 (0)191 334 2114
bInstituto de Ciencia de Materiales de Aragón (ICMA), Universidad de Zaragoza-
CSIC, Departamento de Física de la Materia Condensada, 50009 Zaragoza, Spain.
E-mail: smartins@unizar.es
cLaboratorio de Microscopías Avanzadas (LMA) C/Mariano Esquilor s/n Campus Rio
Ebro, 50018 Zaragoza, Spain
dInstituto de Nanociencia de Aragón (INA) and Departamento de Química Física,
Facultad de Ciencias, Universidad de Zaragoza, 50009 Zaragoza, Spain
eNanomaterials and Nanotechnology Research Center (CINN), University of Oviedo-
CSIC, Departamento de Física, Oviedo, Spain
fDepartment of Chemistry, University of Liverpool, Liverpool, L69 7ZD, UK
†Dedicated to Professor David Cole-Hamilton,
a gentleman scholar of the
highest calibre, and much respected friend, on the occasion of his retirement
and for his outstanding contribution to transition metal catalysis.
‡Electronic supplementary information (ESI) available: Acknowledgements,
details of synthetic procedures and crystallographic structure determinations for
3a and 3b; experimental details of the I(s) measurements; single molecule con-
ductance for 4 and 1a; and XPS measurements. CCDC 893525 and 893526.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt31825c
338 | Dalton Trans., 2013, 42, 338–341
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Fig. 1 Complexes used in this work E = Si (a), C (b).
Blodgett films and in single molecule configurations. Conduc-
tance values obtained from Me3SiCuC/NH2 contacted junc-
tions formed from 1a are of the same order of magnitude as
obtained from thiol–thiol anchored OPEs when measured by
the I(s) method.9 When compared with thiol, –SH, and amine,
–NH2, contacting groups, the additional steric bulk of the
SiMe3 group was thought to be potentially useful in limiting
the range of accessible surface binding sites, which in turn
could be used to give rise to molecular junctions with more
reproducible conductance signatures. To explore the potential
of trimethylsilylethynyl contacting groups in more detail we
have undertaken a study of a series of molecular junctions
formed from OPE and organometallic derivatives featuring
–CuCSiMe3 and –CuCCMe3 contacts using the I(s) method.
The compounds 2a and 3a (Fig. 1 and 2) offer rigid, linear
molecular geometries with estimated Si⋯Si distances of 24.49
(2a)–23.97 (3a) Å.§ 10‡ In contrast to related Pt complexes in
which the metal acts as an insulating fragment,11 these trans-
Ru(CuCR)2(dppe)2-based systems offer a highly delocalised
π–d–π electronic structure which spans the length of the mole-
cular backbone.12 Electrical measurements were performed
using low-coverage of the target molecule on a Au(111) gold
substrate using an STM operating in the I(s) configuration,13‡
with relatively high set point currents (20 nA) although no
special care was taken to record data exclusively from flat
Au(111) terraces to allow formation of both type A and type B
contacts.
Fig. 2 Molecular structure of 3a. Selected bond lengths (Å) and angles (°):
Ru(1)–P(1, 2) 2.3598(6), 2.3555(7); Ru(1)–C(1) 2.066(3); C(1)–C(2) 1.204(4);
C(2)–C(3) 1.438(4); C(6)–C(9) 1.449(4); C(9)–C(10) 1.197 (4); C(10)–Si(1) 1.836(3);
P(1)–Ru(1)–P(1’) 180.0.
Fig. 3 Typical conductance traces from 2a and 3a using the I(s) method and
conductance histograms derived from I(s) measurements. The curves are shifted
horizontally for clarity. Conductance data are presented in units of the conduc-
tance quantum G0 = 2e2/h = 77.5 μS. I0 = 20 nA and Ut = 0.6 V.
Fig. 3 shows typical I(s) curves exhibiting current plateaus
from 2a and 3a. The plateaus are attributed to the formation
of conductive molecular junctions and can be observed in
ca. 14–16% of the scans. Conductance histograms reveal single revealed two conductance peaks for type A ((2.99 0.43) × 10−5
conductance values of (2.75 0.56) × 10−5 G0 (2a) and (5.10
G0) and B ((7.92 1.33) × 10−5 G0) contacts.‡ The similarity of
0.99) × 10−5 G0 (3a) and the exclusive formation of ‘type A’ con- the type A conductance values of 2a and 4 indicates the electri-
tacts (Fig. 3). The higher conductivity of the organometallic cal similarity of the trimethylsilyl and amino contacts to gold.
molecular junction from 3a is consistent with the slightly The type B contact from 1a was less conductive than bis-
shorter length of the molecule, and the better alignment of (amine) contacted 4, but clearly distinguishable from the
the molecular HOMO with the Fermi levels of the gold con- type A peak.
tacts.14 Under the same conditions, the reference compound
Recently, the formation of highly transmissive Au–C con-
4, which contacts to each metallic surface in the junction tacts from addition of Me3Sn–alkyl bonds to gold surfaces
through an amine moiety, gave rise to two well-resolved con- during STM-BJ measurements has been reported.16 Although
ductance peaks due to type A ((3.20 0.83) × 10−5 G0) and type the Si–C bond in trialkylsilyl-ethynyl moieties is sensitive to
B ((14.4 2.78) × 10−5 G0) contacts,‡ the latter in good agree- cleavage following attack at Si by nucleophiles, compounds
ment with data from STM-BJ measurements.15 Conductance containing this moiety are rather more environmentally stable
histograms constructed from I(s) measurements with 1a also than those with trialkyltin functionality and may be less prone
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Dalton Trans., 2013, 42, 338–341 | 339

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to rupture in the presence of the substrate surface. Chemisorp- group in alkynyl chemistry, and the formation of molecular
tion of the –CuCSiMe3 functionalised molecules on gold was junctions with unique conductance profiles using this contact,
demonstrated by Quartz Crystal Microbalance (QCM) experi- the trimethylsilylethynyl moiety holds significant promise as a
ments. AT-cut, α-quartz crystals with a resonant frequency of contacting group. Work is underway to clarify the molecule–
5 MHz having circular gold electrodes patterned on both sides substrate interaction and to confirm the role of the methyl
were incubated in 0.01 mM solutions of 2a and 3a in CHCl3 groups in restricting access to surface defect sites and
for 24 h. Afterwards the substrates were thoroughly rinsed with adatoms.
CHCl3 and the variation of the resonant frequency of the sub-
strates before and after incubation was determined. High
surface coverage of 7.32 × 10−10 and 3.90 × 10−10 mol cm−2
Notes and references
were obtained for 2a and 3a compounds, respectively. XPS
measurements on 2a and 3a as both powders and SAMs on
Au were also undertaken.‡ The Si 2p3/2,1/2 doublet appears as a
single, asymmetric peak due to the small spin–orbit coupling
in Si with BE of 100.86 (2a) and 101.26 (3a) eV, and 151.94 (2a)
and 152.59 (3a) eV for the 2s peak in the powder samples, con-
sistent with the Me3Si–CuC moiety.8,17
The signal intensity is much lower in the SAMs and resol-
ving the surface bound and free SiMe3 moieties is difficult,
with only a single, broad peak shifted with respect to the
powders (2p, 2s: 2a 102.88, 154.31; 3a 103.19, 154.28 eV). The
shift in the 2s peak may be some indication of a change in
hybridisation at Si.
§Crystal data: 3a: C78H74Si2P4Ru, M = 1292.50, triclinic, a = 9.4265(4), b =
13.5130(5), c = 14.2919(6) Å, α = 76.253(2), β = 74.292(3), γ = 71.596(2)°, U =
1639.65(12) Å3, T = 120.0 K, space group PN, Z = 1, μ(MoKα) = 0.417, 18 597 reflec-
tions measured, 8224 unique (Rint = 0.0493) which were used in all calculations.
The final R1(F) = 0.0453 for 6129 reflections with I ≥ 2σ, wR(F2) = 0.1119 (all
data), GOF = 0.990; 3b: C84H78Br0.1Cl12P4Ru, M = 1745.80, triclinic, a = 10.8015(4),
b = 12.4654(4), c = 16.4565(6) Å, α = 94.896(1), β = 105.108(1), γ = 103.004(1)°, U =
3
ˉ
2059.77(13) Å , T = 120.0 K, space group P1, Z = 1, μ(MoKα) = 0.749, 35 554 reflec-
tions measured, 11 468 unique (Rint = 0.0314) which were used in all calcu-
lations. The final R1(F) = 0.0429 for 9600 reflections with I ≥ 2σ, wR(F2) = 0.1141
(all data), GOF = 1.083. Crystallographic data for the structure have been depos-
ited with the Cambridge Crystallographic Data Centre as supplementary publi-
cation CCDC-893525 and 893526.
To further establish the electronic functionality of the silyl
group, molecular junctions featuring –CuCCMe3 terminal
groups 1b, 2b and 3b were examined within an identical I(s)
configuration, but no conductance plateaus could be detected
in any case over 5000 individual measurements per molecule.
The physical and electronic differences between tert-butyl- and
trimethylsilyl-ethynyl groups has also been noted in compari-
sons of the SAM forming behaviour of molecules bearing these
functional groups.8 The molecular structure of 3b displays
little variation from the silyl analogue,‡ but there are substan-
tial differences in physical/electrical behaviour of junctions
formed by trimethylsilylethynyl and tert-butyl ethynyl con-
tacted molecules. It has been proposed that given the propen-
sity for Si(IV) to adopt coordination numbers greater than four,
that the –SiMe3 groups can adopt a five-coordinate, trigonal
bipyramidal geometry with a Au–Si interaction, aided by the
presence of the electron-withdrawing ethynyl substituent.8,18
In this conformation, the steric bulk of the trimethyl groups
may restrict binding at higher coordination surface sites,
resulting in exclusive A-type contacts. In contrast, the tert-butyl
contact can only ‘bind’ to Au via weak and longer-range van
der Waals contacts, leading to ineffective molecule-surface
coupling.
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340 | Dalton Trans., 2013, 42, 338–341
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