Molecular Logic Gates
TiO2-Based Light-Driven XOR/INH Logic
Luis F. O. Furtado, Anamaria D. P. Alexiou,
L?ia Gon?alves, Henrique E. Toma,* and Koiti Araki*
The ever-growing demand for faster and smaller devices has
been successfully fulfilled by the semiconductor industry
through formidable scientific and technological advances in
the manufacturing of ever-smaller silicon-based devices.
However, this astonishing progress is almost reaching its
physical limitsas the size of the components goes further
into the nanoscaleworld, requiring the urgent development of
devices with innovative architectures. Efforts are underway to
identify and develop alternative systems to the current
complementary metal oxide semiconductor (CMOS) tech-
nology, such as quantum cellular automata,single-electron
transistors (SETs),and quantum computingand molec-
Molecular systems are interesting candidates for the
storage, processing, and communication of information/data.
Recently, several examples have been reported that emulate
the functions of memories, transistors, rectifiers, switches, and
wires.[6–9]Great attention is also being given to the construc-
tion of molecular logic gates that mimic the functions of
present silicon devices. Accordingly, several systems using
electrical, chemical, optical, or mechanical signals as inputs
and outputs[5,10–13]have been described that generally involve
complicated arrangements, equipment, or processes.
One of the biggest challenges to the development of a
viable technology based on molecular systems is the external
addressability and compatibility with present-day electronic/
nanoelectronic devices and circuits. Generally, conceptual
models that explore molecular junction properties involve
one or more steps in which a solution is injected into the
system. Consequently, function of the device is compromised
by long washout times. Herein, we report the first molecular
optoelectronic XOR logic gate that operates exclusively with
[*] Dr. L. F. O. Furtado, Dr. A. D. P. Alexiou, L. Gon?alves,
Prof. H. E. Toma, Prof. K. Araki
Instituto de Qu?mica
Universidade de S¼o Paulo
Caixa Postal 26077, CEP 05513–970 S¼o Paulo, SP (Brazil)
[**] This research was supported by Funda?¼o de Amparo ? Pesquisa do
Estado de S¼o Paulo (FAPESP), Conselho Nacional de Desenvolvi-
mento Cient?fico e Tecnol?gico (CT-Energ/CNPq 400.804/2003-4),
Instituto do MilÞnio de Materiais Complexos (IMMC), and Rede de
Nanotecnologia Molecular e Interfaces (RENAMI). We also thank
Degussa for kindly supplying titanium dioxide P-25 and Prof. Ivo A.
H?mmelgen for helpful discussions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2006, 45, 3143–3146? 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
optical inputs and an electric output, making the system easily
addressable and compatible with present electronic systems.
The device is based on a TiO2dye-sensitized Gr?tzel-type
cell,[14,15]which has attracted widespread interest for the
conversion of sunlight into electricity. A typical Gr?tzel cell
consists of a thin nanocrystalline TiO2film on a conductive
glass electrode (fluorine-doped tin oxide, FTO) covered with
a chemisorbed chromophore such as [Ru(dcb)2(SCN)2)]2+
(“N3”; dcb=2,2’-bipyridine-4,4’-dicarboxylic acid). The cir-
cuit is closed and made regenerative by connecting it with a
platinized FTO cathode and filling the space in between them
with a hole-conducting electrolyte such as a solution of I3
in CH3CN. The photosensitizer molecules absorb light and
inject electrons into the n-type nanocrystaline semiconductor
of electric power. However, direct absorption of light by TiO2
(lexc<400 nm; Figure 1B, curve b) also produces a similar
photoinduced charge separation, such that the dye is used to
extend the range of harvested light into the visible region.
By using a dye that accepts rather than injects electrons
into the semiconductor layer when in the excited state, the
direction of the photocurrent output can be modulated by the
wavelength of the incident light, making possible the gen-
eration of a light-driven logic gate. For this purpose, the
molecular dye should be reductively quenched by the nano-
crystalline semiconductor layer. Consequently, the ground-
state reduction potential should be more positive than about
0.0 V to satisfy this requirement when 2.5-eV photons (lexc
?500 nm) are used for excitation, as the energy of the TiO2
valence band (upper limit) and the standard hydrogen
electrode referenced to vacuum level are 6.8and 4.44 eV,
respectively. Generally, the reduction potential of organic
compounds and ruthenium polypyridine complexes such as
N3 is more negative than ?1.0 V, and they are well-known
electron injectors. However, ruthenium acetate trinuclear
clusters reveal reduction potentials that lie in a suitable range.
For example, the complex [Ru3O(Ac)6(py)2(pzCO2H)]PF6(1;
Ac=acetyl, py=pyridine, pz=pyrazine) exhibits favorable
characteristics and was used to demonstrate our hypothesis
(see Figure 1 and Supporting Information).
Recently, Biancardo et al.reported a photonic NOR
gate based on a TiO2photoelectrochemical cell. However,
their cell was based on a completely different design and
concept in which potential and CuIIions were used as inputs
and the luminescence of [Ru(dcb)2(CN)2] dye was used as the
The photoaction spectrum of a cell assembled with the
ruthenium cluster dye is shown in Figure 1B, curve a. A
photocurrent maximum is observed around l=350 nm,
passes through zero at 375 nm, and remains negative up to
l=600 nm, with a peak negative photocurrent value at
410 nm, where the absorption band of the adsorbed 1 is
localized (Figure 1B, curve d). Note that the spectrum of the
sensitized TiO2 film is similar to that of the sum of the
nanocrystalline semiconductor and complex 1 and that the
contribution of this species decreases below l=400 nm,
indicating that at least two processes are driving the photo-
current in opposite directions as shown in Figure 2. One is the
direct absorption of light by the n-type TiO2layer and the
second is the abstraction of electrons by the excited cluster
complex.The first process ispredominant atlexc<375 nm and
the current should increase sharply as lexcdecreases, but the
filtering effect by glass also increases rapidly, giving rise to the
maximumat 350 nm (Figure 2A). Thispeak isthe sole feature
observed in a similar device assembled with a nonsensitized
nanocrystalline TiO2film. Above 375 nm the photocurrent
becomes dependent on the excited-state properties of the
adsorbed dye. The adsorbed complex 1 displays a broad
charge-transfer band at 420 nm (Figure 1B, curves c and d), at
the negative peak in the photoaction spectrum, but the
intracluster band at 700 nm is the most intense one. Electron
injection from “hot” excited states are known to be respon-
sible for the high efficiency of Gr?tzel cells, but this is the first
report on electron injection in the reverse direction, that is,
from the TiO2film to the excited dye sensitizer.
The excited complex 1* (E*(1*/1+) for oxidation/electron
injection to TiO2 and E*(1*/1?) for reduction/electron
removal from TiO2) can inject electrons to the semiconductor
layer, but this cannot be the main process because current
flows in the opposite direction (Figure 2B). Also, a mecha-
nism in which there is first oxidative quenching of 1* by I2was
also ruled out because TiO2cannot reduce the 1+species.
Figure 1. A) Diagram showing the energy levels of TiO2and the Ru
cluster complex adsorbed on the surface (the asterisk represents an
excited complex). The energy levels of the ground (E(E)=Eredox+
4.44 eV) and excited states (E(E*)=Eredox?Ehn+4.44 eV) referenced to
the vacuum level were calculated from the redox potentials. CB=con-
duction band; VB=valence band. B) a) Photoaction response of the
assembled device; b,c) reflectance spectra of the nanocrystalline TiO2
film on FTO (b) and in the presence of a cluster sensitizer (c); and
d) reflectance spectrum of a solid (powder) cluster complex.
? 2006 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimAngew. Chem. Int. Ed. 2006, 45, 3143–3146
Consequently, the reverse photocurrent should be generated
by the mechanism depicted in Figure 2A, involving the
reductive quenching of 1* by the TiO2layer, followed by
reduction of I2to I?by 1?species and migration of electrons
to the counterelectrode, closing the circuit.
Such a photoelectrochemical response can be readily
exploited in the design of a XOR logic gate, which has proven
difficult to emulate on the molecular scale.[17,18]Generally,
these devices have two inputs and one output that follow the
truth table shown in Figure 3A. The photocurrent output
(Out) obtained by exciting with light at 350 and 425 nm as
inputs (In1 and In2, respectively) are shown in Figure 3B.
When In1 and In2 are equal to 0 (i.e. no light, 0-0), no
photocurrent is generated (Out=0). When lexc=425 nm (0-
1), a cathodic photocurrent is recorded and Out=1. An
anodic photocurrent is recorded when light at 350 nm hits the
cell (1-0) and again Out=1. However, if the cell is irradiated
simultaneously at both excitation wavelengths (i.e. 350 and
420 nm, 1-1), no net current flows as they are canceled out
(Out=0). The truth table matches that for the XOR gate, as
shown in Table 1. Once the binary logic is based on threshold
values,the INH gate also can be obtained simply by
changing the intensity or the excitation wavelength of one of
the inputs in such a way as to decrease the absolute output
signal and establish a threshold in between Out1 and Out2.
For example, one can set 7 and 13 mA as the outputs and
10 mA as the threshold to realize the INH logic gate, whose
logic responses are summarized in Table 1.
In summary, optoelectronic XOR and INH logic gates
based on two light inputs and a current output were
demonstrated for the first time by using a nanocrystalline
TiO2 dye-sensitized solar Gr?tzel-type cell with reverse
injection of electrons at the semiconductor/dye interface.
These findings should give helpful insights for the develop-
ment of new light-driven devices and the realization of more
intricate systems. Further investigations in this direction are
[Ru3O(Ac)(py)2(pzCO2H)]PF6(1) was obtained from treatment of
[Ru3O(Ac)6(py)2(CH3OH)]PF6(2), which was prepared according to
an adapted procedure,with carboxypyrazinic acid (pzCO2H) in
CH3OH/CH2Cl2(1:5 v/v). The reaction mixture was left for 20 h at
258 8C, filtered free of any solids, and added dropwise into diethyl
ether. The resultant precipitate was purified by multiple recrystalli-
Figure 2. Scheme showing the mechanisms of photocurrent generation
with inputs of A) 1-0 and B) 0-1. See text for details.
Figure 3. A) Symbolic representation and truth table for the XOR and
INH logic gates. B) Photoresponses for the combinations of inputs In1
and In2 corresponding to the XOR logic-gate device.
Table 1: Summary of the response of the optoelectronic device corre-
sponding with the truth table of XOR and INH devices shown in
425 and 350 nm
[a] In1: lexc=350 nm; In2: lexc=425 nm.
Angew. Chem. Int. Ed. 2006, 45, 3143–3146? 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
zation steps in CH2Cl2/acetone mixtures to afford dark green crystals Download full-text
(0.33 g, 42%); elemental analysis (%) calcd for Ru3C27H38O15N4: C
29.46, H 2.93, N 5.09; found: C 29.43, H 3.02, N 5.08.
The photoelectrochemical cell was prepared according to the
therein).[15,20]A viscous dispersion obtained by grinding Degussa
P25 TiO2nanoparticles (30% rutile/70% anatase, mean particle size:
30 nm) with Triton-X 100 and acetylacetone, was spread onto a
fluorine-doped SnO2 conducting glass slide (TEC15 FTO glass,
?15 Wcm?2). The film was deposited over a 1-cm2area, limited by
the use of Scotch tape on the center of a 2 cm?2 cm square piece.
After drying, the samples were fired at 4508 8C in air to give films with
an average thickness of 14 mm. After cooling, the electrode was
soaked in a solution of 1 (1x10?4m) in acetonitrile for 6 h and
assembled with a Pt-coated FTO glass counterelectrode form a
sandwich-type cell. The circuit was closed by filling the space between
the electrodes with an electrolyte solution containing I2(0.05m),
tetrabutylammonium iodide (0.8m), LiI (0.1m), and tert-butylpyridine
(0.5m) in acetonitrile.
The photoaction spectrum was recorded using an Oriel Spectral
Luminator (model 69050) as light source and a Hung Chang Building
CDM digital multimeter (model HC608).
The logic gates were demonstrated by using the Spectral
Luminator and an Applied Photophysics 150-W Xe lamp with a
Czern-Turner monochromator as light sources, irradiating at 350 and
425 nm, respectively.The lightfromtheformerwasbroughttothe cell
by using a bundle fiber-optics cable (f=3 mm) in a 458 8 arrangement.
was 2.9 (350 nm) and 4.3 mWcm?2(420 nm). The output was
measured with the multimeter as before. A much simpler arrange-
ment using light sources with band-pass filters can also be used.
and co-workers(method B
Received: January 9, 2006
Revised: February 13, 2006
Published online: March 30, 2006
optoelectronics · sensitizers · titanium
Keywords: molecular devices · nanotechnology ·
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