A three-valued photoelectrochemical logic device realising accept anything and consensus operations

Article abstract of DOI:10.1039/c4cc09980j

A new application of a hybrid material exhibiting the photoelectrochemical photocurrent switching (peps) effect in a three-valued logic device is reported. In contrast to other similar peps-based systems, the one described here is capable of performing basic ternary logic operations: gullibility and consensus. This journal is

Full text of DOI:10.1039/c4cc09980j

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A three-valued photoelectrochemical logic device
realising accept anything and consensus
operations†
Cite this:DOI: 10.1039/c4cc09980j
Received 15th December 2014,
Accepted 20th January 2015
M. Warzecha,a M. Oszajca,b K. Pilarczyk*cd and K. Szaciłowski*de
DOI: 10.1039/c4cc09980j
www.rsc.org/chemcomm
A new application of a hybrid material exhibiting the photoelectro- devices and generated photocurrent pulses in response to the
chemical photocurrent switching (peps) effect in a three-valued logic optical stimulation, but its function could be only described as a
device is reported. In contrast to other similar peps-based systems, three-state switch or a binary-to-ternary converter. This communica-
the one described here is capable of performing basic ternary logic tion presents the first case of Boolean ternary logic operation
operations: gullibility and consensus.
performed in the photoelectrochemical device.
Wide band gap semiconductors, including titanium dioxide,
The research in molecular-scale logic devices has already reached generate photocurrent upon excitation with light of appropriate
a certain level of maturity. This odyssey started with the seminal wavelength. Its direction (polarity) depends on the doping state
Nature paper by A. P. de Silva1 and within twenty years evolved of the semiconductor: n-type semiconductors generate typically
into a rapidly developing field. There are hundreds of examples anodic photocurrents, whereas the p-type semiconductors generate
of all possible logic gates and switches, as well as arithmetic the cathodic ones. However, when the surface of semiconductor is
circuits, multiplexers and demultiplexers, encoders and even modified with the molecular species exhibiting significant light
simple cryptographic devices.2–7 Nonetheless, two fields, whose absorption (usually at the lower energies than the band gap width)
roles are crucial for information processing, have not been they may inject electrons into the conduction band, or holes into
explored so far. One of them is reversible logic. The molecular- the valence band of the semiconductor. This process, called photo-
scale reversible logic gate, the so called Feynman gate was sensitization, is commonly utilised in dye-sensitized solar cells. If the
reported only once by U. Pischel in 2009.8 The other concept, surface molecule is also redox-active and an appropriate sacrificial
which did not arouse sufficient interest, is that of multivalued reagent is present in the solution, the process of photosensitization
and fuzzy logic. Although, these approaches have already been becomes redox-controlled, which gives rise to various switching
implemented in molecular-scale devices, they are capable of phenomena.16 These effects can be used in the implementation of
operating only in solutions: three-valued logic by U. Pischel8 numerous binary logic functionalities.17,18
and fuzzy logic by P. L. Gentili.9–14 The first attempt towards the
Here we report the use of certain photoelectrochemical
introduction of ternary logic in solid state systems utilising the properties of titanium dioxide modified with hexacyanobutadienide
photoelectrochemical photocurrent switching effect (the PEPS (1) and hexacyanodiazahexadienide (2) anions (Scheme 1) in a
effect) was presented in 2011 by Oszajca et al.15 The system ternary logic device. The parent compounds, prepared using a
worked in an aqueous electrolyte, like almost all PEPS-based well-established synthetic route from tetracyanoethene, undergo
strong adsorption at the surface of n-type titanium dioxide with
a Strathclyde Institute of Pharmacy and Biomedical Sciences,
University of Strathclyde, 161 Cathedral Street, Glasgow, UK
b Department of Chemistry and Applied Biosciences, ETH Zurich,
Vladimir-Prelog-Weg 1, 8093 Zurich, Switzerland
subsequent reduction by conduction band electrons, originating
c AGH University of Science and Technology,
Faculty of Physics and Applied Computer Science, al. A. Mickiewicza 30,
´
30-059 Krakow, Poland. E-mail: kacper.pilarczyk@fis.agh.edu.pl
d AGH University of Science and Technology, Academic Centre for Materials and
´
Nanotechnology, al. A. Mickiewicza 30, 30-059 Krakow, Poland
e AGH University of Science and Technology, Faculty of Non-Ferrous Metals,
´
al. A. Mickiewicza 30, 30-059 Krakow, Poland. E-mail: szacilow@agh.edu.pl
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc09980j
Scheme 1 The structures of cyanocarbon modifiers.
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from oxygen vacancies. Such a process, which involves the with-
drawal of charge carriers from the semiconductor is usually
called surface transfer doping19 and has been recently observed
in chromate(VI) and fluorochromate(VI)-modified titania.20
The spectroscopic measurements indicate that the modifiers
are adsorbed in the anionic form, and a new absorption band
ranging from 2.4 to 3.0 eV can be observed. This transition can
be associated with a charge transfer process involving surface
Ti(IV) centres and adsorbed cyanocarbon anions.
The photoelectrochemical data suggest that novel materials
show only minor photosensitization with photocurrents generated
with the excitation wavelength from 300 to 525 nm. Nonetheless,
the photocurrent switching effect is strongly pronounced. The
anodic photocurrents are recorded only within a 300–400 nm
Fig. 2 The mechanism of the photocurrent generation under anodic (a),
mixed (b) and cathodic conditions (c). X stands for the surface molecule.
window and at the positive potential of the photoelectrode, which Therefore TiO2 with surface modified with 1 or 2 can be regarded as
is a typical behaviour observed for titanium dioxide. At the same a hybrid material combining some features of n-type and p-type
time, the change of the polarization leads to the generation of semiconductors characterised by significantly different band gaps
cathodic photocurrents – at first only within the 400–500 nm range (Fig. 2).
(together with the anodic photocurrent) but at the lower photo-
electrode potentials only the cathodic photocurrents are observed to the Butler–Volmer equation, the p-type semiconductor cannot
(Fig. 1). generate any photocurrent. The same situation occurs with the
At the potentials higher than the switching potential, according
This peculiar behaviour (quite different from that of other more negative polarization and anodic photocurrents. Hence, one
PEPS-based systems) can be explained by the presence of two may conclude that a subtle competition between two processes
independent photocurrent generators. The inner parts of the (Fig. 2b) governs the overall polarity of the photocurrent generated
TiO2 nanoparticles retain their n-type character and are responsible at the modified electrodes and that in the intermediate state no net
for anodic photocurrent generation. On the other hand, the surface photocurrent is generated (which corresponds to the unknown
transfer doping with 1 or 2 results in a significant decrease in the state of the device). Furthermore, different surface states induce
conduction band electrons concentration, which is reflected by a fine variations of surface molecules redox properties, leading to
strong increase of the band bending at the interface, and con- slightly different frontier orbital energies; hence the wavelength
tributes to the widening of the depletion layer at the outer part of dependence is observed.
the particles (Fig. S1 and S2, ESI†). Such a mechanism is consistent
It can be noticed that the photocurrent action map can be
with the observed photocurrent profile and is also supported by the divided into three distinct regions according to the photocurrent
data published on strong electron acceptors adsorbed at n-type polarity and intensity: anodic (with dominating anodic photo-
semiconductors, where surface hydroxyl ions as well as oxygen current), cathodic (with dominating cathodic photocurrent) and
vacancies can serve as electron donors.21–23 In the case of cyano- null photocurrent area, where anodic and cathodic photo-
methylene compounds the obtained surface species behave as currents effectively compensate each other.
efficient chromophores, absorbing in the visible light region, which
It appears immediately that such a behaviour corresponds to
can be involved in the photoinduced electron transfer processes. a three-state logic circuit with À1, 0 and 1 (FALSE, UNKNOWN,
TRUE) outputs in a very natural way: positive (anodic) photocurrent
corresponds to logical ‘‘1’’, negative (cathodic) photocurrent to
logical ‘‘À1’’, whereas areas with no net photocurrent to logical
‘‘0’’. Furthermore, individual ranges of the applied potential and the
incident light wavelength can be assigned to ternary Boolean values,
as indicated in Fig. 3 (cf. Fig. S4 (ESI†) for compound 2).
A simple analysis leads to the conclusion that the photo-
electrodes composed of cyanocarbon-modified titanium dioxide
behave like ternary (three-valued) logic devices. These devices
(and their functions) may be described in terms of one of the
basic operators – the accept anything (or gullibility) operator or
consensus operator.24
The gullibility operator returns UNKNOWN output either
with two UNKNOWN inputs or when two inputs are of opposite
character (one TRUE and one FALSE). In other cases it yields an
output equal to any non-UNKNOWN input. The truth table
presented above (Fig. 4a) exactly matches the one which is
in the presence of oxygen in 0.1 M KNO3 (cf. Fig. S3 (ESI†) for compound 2). proposed in the photocurrent action map (Fig. 3a).
Fig. 1 The photocurrent action spectra recorded for TiO2 modified with 1
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Communication
In the presented communication we show the application of
simple hybrid materials (based on the cyanocarbon-modified
titanium dioxide) in the construction of three-valued photo-
electrochemical logic devices, which realise the gullibility and
consensus operations. This is the unique case, where two dual
operators can be realised in the same chemical system just by a
simple change in the output threshold values. We also propose
the mechanism responsible for the properties of the investigated
system along with the interpretation of the recorded photo-
current action spectra in terms of ternary Boolean logic. Such
systems may become fundamental to multivalued optoelectronic
logic circuits based on the easily accessible hybrid materials and may
contribute to the development of this certainly underappreciated
field of information processing.
Fig. 3 A fragment of a photocurrent action map with the three-valued
truth table corresponding to the accept anything operation (a). The
consensus operation can be realized by neglecting the photocurrents
with lowest intensities (b).
Financial support from the National Science Centre (grant no.
UMO-2011/03/B/ST5/01495) and the Foundation for Polish Science
(grant no. 71/UD/SKILLS/2014) is gratefully acknowledged.
Notes and references
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Fig. 4 The truth tables and symbols for the ternary gullibility (a) and
consensus (b) operations.
´
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A simple modification of the photocurrent threshold, which
neglects some isovalue lines adjoining the UNKNOWN area
leads to another ternary basic operator – consensus. The consensus
operation applied to two ternary variables returns FALSE if both
inputs are FALSE, TRUE if both are TRUE or UNKNOWN in all the
other cases. These two gates of a dual nature are natural extensions
of OR and AND gates in three-valued logic. At the same time, the
system is incapable of realising the negation operation (the ternary
analogue of NOT); hence a complete set of logic operators cannot
be achieved.
The consensus and gullibility operations are rarely used in the
design of ternary logic circuits but are extremely useful in the
construction of arithmetic devices based on the balanced ternary logic.
Moreover, contrary to the molecular logic gates operating in solution,
the presented system can be easily concatenated.25 The output
(current) is compatible with one of the inputs (voltage) and an external
resistor would be sufficient for the proper communication between
individual gates. Similar solution can be used to prevent bidirectional
information transfer which could lead to undesired feedback loop – a
simple addition of a Schottky diode (e.g. based on a thin CdS layer)25
would provide unidirectional information transfer. Since the system is
based on the interaction with light (from an external source), the signal
amplification is not required and the photocurrent amplitude may be
sustained at the constant level within a circuit.
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Chem. Commun.