Molecular Quantum Dot Cellular Automata (mQCA) are among the most promising emerging technologies for the expected theoretical operating frequencies (THz), the high device densities and the non-cryogenic working temperature. In this paper, we performed an analysis of the possible fabrication defects of a molecular QCA wire built with ad-hoc synthesized bis-ferrocene molecules. We then evaluated the fault tolerance of a real QCA device and assessed its performance in non-ideal conditions, by defining a new methodology for the fault analysis in the mQCA technolog

Graziano, Mariagrazia

Piccinini, Gianluca

Pulimeno, Azzurra

Wang, R.

PORTO Publications Open Repository TOrino

Politecnico di Torino
Porto Institutional Repository
[Proceeding] Fault Tolerance Analysis of a Bis-Ferrocene QCA Wire
Original Citation:
Pulimeno A.; Graziano M.; Wang R.; Piccinini G. (2013). Fault Tolerance Analysis of a Bis-
Ferrocene QCA Wire. In: Workshop on Design and Test Methodologies for Emerging Technologies
(DETMET), Avignon, France, 2013.
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Fault tolerance analysis of a bis-ferrocene QCA wire
Azzurra Pulimeno∗, Mariagrazia Graziano∗, Ruiyu Wang∗ and Gianluca Piccinini∗
∗Department of Electronics and Telecommunications, Politecnico di Torino, Italy
Email: {azzurra.pulimeno, mariagrazia.graziano, gianluca.piccinini}@polito.it
Abstract—Molecular Quantum Dot Cellular Automata
(mQCA) are among the most promising emerging technologies
for the expected theoretical operating frequencies (THz), the high
device densities and the non-cryogenic working temperature. In
this paper, we performed an analysis of the possible fabrication
defects of a molecular QCA wire built with ad-hoc synthesized
bis-ferrocene molecules. We then evaluated the fault tolerance
of a real QCA device and assessed its performance in non-ideal
conditions, by defining a new methodology for the fault analysis
in the mQCA technology.
I. INTRODUCTION
The reason at the root of the expectations on Quantum Dot
Cellular Automata devices as successful CMOS substitutes [1]
is that no charge transport is involved, but only local field inter-
action determines the information transfer. Power consumption
is then expected to be dramatically reduced. According to the
Lent theory [2], the smallest and most performing QCA cell
could be implemented with a molecular system. The simplest
demonstration of logic propagation that could be performed
according to this paradigm is a molecular wire (see Fig.
1(B)). In [3], [4] some possible candidate molecules were
presented: all these molecules are ideal systems and are studied
in vacuum, so they are not suitable for a real circuit. In
this work we addressed our fault tolerance analysis to a real
molecule, the bis-ferrocene (see Fig. 1(A)), synthesized ad hoc
for QCA purpose [5], [6]. Both the QCA properties and the
fault tolerance of this molecule have been already proved in
[6], [7], [8].
II. DEFECT MODELS
In order to experimentally demonstrate the QCA function-
alities, we are preparing a bis-ferrocene molecular wire like the
one in Fig. 1(B), based on a gold nanowire upon which the
molecules can be binded through the thiol element. During
the gold substrate preparation and the molecule deposition,
some defects may occur and they are schematically sketched
in Fig. 2: i) misalignment of deposited molecules, due to the
defects of the grain or the irregularity of the hexane-dithiol
monolayer (Fig. 2(A)); ii) huge vertical distance between two
dots, caused by the roughness of the gold substrate (Fig. 2(B));
iii) variations of the distance of two nearby molecules, in
case of many hexane-dithiol elements between consecutive
molecules (Fig. 2(B)). These defects might cause faults and
mis-behaviors of the cells in the molecular QCA wire.
In order to evaluate the fault tolerance of a molecular
QCA wire, we defined a methodology focused on a system
composed of an ideal emulated driver, a bis-ferrocene molecule
under test (MUT) and an ideal receiver, as part of the molecular
wire. The state of the driver should localize the MUT charge
on one of the active dots. Referring to the above mentioned
x
z
y
carbazole
ferrocenes
Dot1
Dot3
thiol
1.0 nm
1
.0
 n
m
Dot2
opposite
charge
distance d
   between
     molecules
Dot1
Dot2 Dot3
binding element
gold nanowire(A) (B)
Fig. 1. (A) Bis-ferrocene molecule: two ferrocenes (active dots) are linked
together by means of a carbazole central group (central third dot). An
alkyl-chain and an ending thiol (-SH) group provide the binding to a gold
substrate. (B) Molecular QCA wire: bis-ferrocene molecules aligned over a
gold nanowire.
Fig. 2. Fabrication defects in the case of a QCA wire made of bis-ferrocene
molecules. (a) Top view of a gold grain after molecule deposition: mis-
alignment and tilt defect are sketched. (b) Section of a bis-ferrocene SAM
on gold: vertical shifts may occur due to the roughness inside a gold grain
or at the interface of two grains; higher molecule-molecule distance is caused
by the number of hexane-dithiol elements.
defects classification, we modeled the three types of defects
varying the position of the driver, as sketched in Fig. 3: firstly
we considered the misalignment of the driver with respect to
the two dots (delta X, Fig. 3(A)), due to the defects in the
orientation of the gold grain (see Fig. 4(A)). Then, we varied
the distance between the driver and the MUT (delta Y) to
model a non ideal squared cell (Fig. 3(B)) in case of different
number of spacers between two molecules as depicted in Fig.
2(B). Finally, the roughness of gold grain is modeled as the
shift of the driver along the vertical axes of the MUT as shown
in Fig. 3(C) (delta Z).
Since the charge localization inside the molecule is the
main requirement for QCA bistability validation, we need to
evaluate the charge distribution of the MUT under different
bias conditions (different positions of the driver).
III. RESULTS AND CONCLUSIONS
Fig. 9 displays the results for the driver shifting along the
vertical axes, defines as delta Z. The graph includes the results
for three different distances from the MUT (equivalent to three
dDot1
Dot2
y
x
z
Fig. 3. Three possible fabrication defects are modeled moving the driver
with the respect to its ideal position: (A) the driver is not perfectly aligned
with the two dots, (B) it is placed at a distance different from the ideal d, (C)
the displacement is along the vertical axes of the molecule.
values of delta Y), considering the driver logic state at 1. The
charge difference between the two dots of the MUT decreases
when the value of delta Z is incremented, which is enhanced
for longer distances from the molecule (higher delta Y). In
particular, in the range related to the roughness of the gold
substrate inside a grain (±0.2 ÷ 0.4nm) the molecule still
works properly, that means that the free positive charge is
mainly localized on one of the two dots, encoding a valid
logic state. On the contrary, when the driver-MUT interaction
is at the interface between two gold grains (equivalent to
deltaZ = ±2.0nm) the molecule is in an undefined state,
because the charge of the two dots is almost the same.
 0
 0.1
 0.2
 0.3
 0.4
 0.5
 0.6
 0.7
 0.8
 0.9
-2 -1.5 -1 -0.5  0  0.5  1  1.5  2
M
UT
 a
gg
re
ga
te
d 
ch
ar
ge
 [e
]
delta Z  [nm]
Driver logic state 1, shifted of deltaZ,deltaY
Dot 1
Dot 2
delta Y -0.20 nm
delta Y 0 nm
delta Y +0.25 nm
Fig. 4. Dot charges as function of the driver shifting along the vertical axes
(delta Z) for three different distances from the molecule (delta Y).
Regarding the driver misalignment with the respect to
the two dots of the MUT (delta X), the trend of the dot
charges is reported in Fig. 5 for the ideal distance between the
MUT and the driver (delta Y = 0). In this case, we reported
simultaneously both the driver logic state at 1 and at 0, in order
to check immediately the MUT bistability for a specific driver
position. In particular, for a given delta X value it is possible
to check if the positive charge inside the MUT moves between
the two dots when the driver changes its state from 1 to 0 and
the difference between them is great enough to be considered
a valid state. Figure 5 reveals that in some cases of driver
misalignment the interaction with the MUT is still preserved.
The results we obtained in this work are fundamental to
address the fabrication of a molecular QCA wire on gold. In
particular, the fault tolerance analysis discussed here highlights
the feasibility and functionality of the MQCA wire also in
presence of several defects we faced during our experimental
 0
 0.1
 0.2
 0.3
 0.4
 0.5
 0.6
 0.7
 0.8
-0.4 -0.2 0 0.2 0.4
M
UT
 a
gg
re
ga
te
d 
ch
ar
ge
 [e
]
delta X  [nm]
Driver shifted of delta X @ deltaY 0 nm 
Driver
@ logic 0
Driver
@ logic 1
Dot1
Dot2
Fig. 5. Dot charges as function of the driver misalignment with the respect
of the two dots (delta X) for a fixed molecule-molecule distance (delta Y=0)
processes. These results are promising for the development
of molecular QCA circuits and, at the same time, give an
important feedback to improve the technological process.
REFERENCES
[1] Semiconductor Industry Association, International Technology Roadmap
of Semiconductors, 2010 Update, Emerging research Device (ERD).
[Online]. Available: http://public.itrs.net, 2010.
[2] C.S. Lent, P.D. Tougaw, W. Porod, and G. Bernstein, Quantum cellular
automata, Nanotechnology, vol. 4, pp. 49-57, 1994
[3] C. S. Lent, B. Isaksen and M. Lieberman, Molecular Quantum-Dot
Cellular Automata, J. Am. Chem. Soc., vol. 125, pp. 1056-1063, 2003
[4] C.S. Lent and B. Isaksen, Clocked molecular quantum-dot cellular
automata, Electron Devices, IEEE Transactions on, vol.50, no.9, pp.
1890-1896, Sept. 2003
[5] L. Zoli, PhD Dissertation, Universita´di Bologna, 2010.
[6] V. Arima, M. Iurlo, L. Zoli, S. Kumar, M. Piacenza, F. Della Sala, F.
Matino, G. Maruccio, R. Rinaldi, F. Paolucci, M. Marcaccio, P.G.Cozzi
and A.P. Bramanti, Toward quantum-dot cellular automata units:
thiolated-carbazole linked bis-ferrocenes, Nanoscale, vol.4, pp.813-823,
2012.
[7] A. Pulimeno, M. Graziano, C. Abrardi, D. Demarchi, and G. Piccinini,
Molecular QCA: A write-in system based on electric fields, IEEE
Nanoelectronics Conference (INEC), vol., no., pp.1-2, June 2011
[8] A. Pulimeno, M. Graziano, A. Sanginario, V. Cauda, D. Demarchi, and
G. Piccinini, Bis-ferrocene molecular QCA wire: ab-initio simulations of
fabrication driven fault tolerance, IEEE Transactions on Nanotechnol-
ogy, in press.


Fault Tolerance Analysis of a Bis-Ferrocene QCA Wire

Molecular Quantum Dot Cellular Automata
(mQCA) are among the most promising emerging technologies
for the expected theoretical operating frequencies (THz), the high
device densities and the non-cryogenic working temperature. In
this paper, we performed an analysis of the possible fabrication
defects of a molecular QCA wire built with ad-hoc synthesized
bis-ferrocene molecules. We then evaluated the fault tolerance
of a real QCA device and assessed its performance in non-ideal
conditions, by defining a new methodology for the fault analysis
in the mQCA technology

PULIMENO, AZZURRA

GRAZIANO, MARIAGRAZIA

PICCININI, GIANLUCA

Wang R.

PORTO@iris (Publications Open Repository TOrino - Politecnico di Torino)

04 August 2020POLITECNICO DI TORINORepository ISTITUZIONALEFault Tolerance Analysis of a Bis-Ferrocene QCA Wire / Pulimeno A.; Graziano M.; Wang R.; Piccinini G.. -ELETTRONICO. - (2013). ((Intervento presentato al convegno Workshop on Design and Test Methodologies forEmerging Technologies (DETMET) tenutosi a Avignon, France nel 2013.OriginalFault Tolerance Analysis of a Bis-Ferrocene QCA WireieeePublisher:PublishedDOI:Terms of use:openAccessPublisher copyrightcopyright 20xx IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all otheruses, in any current or future media, including reprinting/republishing this material for advertising or promotionalpurposes, creating .(Article begins on next page)This article is made available under terms and conditions as specified in the  corresponding bibliographic description inthe repositoryAvailability:This version is available at: 11583/2510681 since:IEEE - INST ELECTRICAL ELECTRONICS ENGINEERS INCFault tolerance analysis of a bis-ferrocene QCA wireAzzurra Pulimeno∗, Mariagrazia Graziano∗, Ruiyu Wang∗ and Gianluca Piccinini∗∗Department of Electronics and Telecommunications, Politecnico di Torino, ItalyEmail: {azzurra.pulimeno, mariagrazia.graziano, gianluca.piccinini}@polito.itAbstract—Molecular Quantum Dot Cellular Automata(mQCA) are among the most promising emerging technologiesfor the expected theoretical operating frequencies (THz), the highdevice densities and the non-cryogenic working temperature. Inthis paper, we performed an analysis of the possible fabricationdefects of a molecular QCA wire built with ad-hoc synthesizedbis-ferrocene molecules. We then evaluated the fault toleranceof a real QCA device and assessed its performance in non-idealconditions, by defining a new methodology for the fault analysisin the mQCA technology.I. INTRODUCTIONThe reason at the root of the expectations on Quantum DotCellular Automata devices as successful CMOS substitutes [1]is that no charge transport is involved, but only local field inter-action determines the information transfer. Power consumptionis then expected to be dramatically reduced. According to theLent theory [2], the smallest and most performing QCA cellcould be implemented with a molecular system. The simplestdemonstration of logic propagation that could be performedaccording to this paradigm is a molecular wire (see Fig.1(B)). In [3], [4] some possible candidate molecules werepresented: all these molecules are ideal systems and are studiedin vacuum, so they are not suitable for a real circuit. Inthis work we addressed our fault tolerance analysis to a realmolecule, the bis-ferrocene (see Fig. 1(A)), synthesized ad hocfor QCA purpose [5], [6]. Both the QCA properties and thefault tolerance of this molecule have been already proved in[6], [7], [8].II. DEFECT MODELSIn order to experimentally demonstrate the QCA function-alities, we are preparing a bis-ferrocene molecular wire like theone in Fig. 1(B), based on a gold nanowire upon which themolecules can be binded through the thiol element. Duringthe gold substrate preparation and the molecule deposition,some defects may occur and they are schematically sketchedin Fig. 2: i) misalignment of deposited molecules, due to thedefects of the grain or the irregularity of the hexane-dithiolmonolayer (Fig. 2(A)); ii) huge vertical distance between twodots, caused by the roughness of the gold substrate (Fig. 2(B));iii) variations of the distance of two nearby molecules, incase of many hexane-dithiol elements between consecutivemolecules (Fig. 2(B)). These defects might cause faults andmis-behaviors of the cells in the molecular QCA wire.In order to evaluate the fault tolerance of a molecularQCA wire, we defined a methodology focused on a systemcomposed of an ideal emulated driver, a bis-ferrocene moleculeunder test (MUT) and an ideal receiver, as part of the molecularwire. The state of the driver should localize the MUT chargeon one of the active dots. Referring to the above mentionedxzycarbazoleferrocenesDot1Dot3thiol1.0 nm1.0 nmDot2oppositechargedistance d   between     moleculesDot1Dot2 Dot3binding elementgold nanowire(A) (B)Fig. 1. (A) Bis-ferrocene molecule: two ferrocenes (active dots) are linkedtogether by means of a carbazole central group (central third dot). Analkyl-chain and an ending thiol (-SH) group provide the binding to a goldsubstrate. (B) Molecular QCA wire: bis-ferrocene molecules aligned over agold nanowire.Fig. 2. Fabrication defects in the case of a QCA wire made of bis-ferrocenemolecules. (a) Top view of a gold grain after molecule deposition: mis-alignment and tilt defect are sketched. (b) Section of a bis-ferrocene SAMon gold: vertical shifts may occur due to the roughness inside a gold grainor at the interface of two grains; higher molecule-molecule distance is causedby the number of hexane-dithiol elements.defects classification, we modeled the three types of defectsvarying the position of the driver, as sketched in Fig. 3: firstlywe considered the misalignment of the driver with respect tothe two dots (delta X, Fig. 3(A)), due to the defects in theorientation of the gold grain (see Fig. 4(A)). Then, we variedthe distance between the driver and the MUT (delta Y) tomodel a non ideal squared cell (Fig. 3(B)) in case of differentnumber of spacers between two molecules as depicted in Fig.2(B). Finally, the roughness of gold grain is modeled as theshift of the driver along the vertical axes of the MUT as shownin Fig. 3(C) (delta Z).Since the charge localization inside the molecule is themain requirement for QCA bistability validation, we need toevaluate the charge distribution of the MUT under differentbias conditions (different positions of the driver).III. RESULTS AND CONCLUSIONSFig. 9 displays the results for the driver shifting along thevertical axes, defines as delta Z. The graph includes the resultsfor three different distances from the MUT (equivalent to threedDot1Dot2yxzFig. 3. Three possible fabrication defects are modeled moving the driverwith the respect to its ideal position: (A) the driver is not perfectly alignedwith the two dots, (B) it is placed at a distance different from the ideal d, (C)the displacement is along the vertical axes of the molecule.values of delta Y), considering the driver logic state at 1. Thecharge difference between the two dots of the MUT decreaseswhen the value of delta Z is incremented, which is enhancedfor longer distances from the molecule (higher delta Y). Inparticular, in the range related to the roughness of the goldsubstrate inside a grain (±0.2 ÷ 0.4nm) the molecule stillworks properly, that means that the free positive charge ismainly localized on one of the two dots, encoding a validlogic state. On the contrary, when the driver-MUT interactionis at the interface between two gold grains (equivalent todeltaZ = ±2.0nm) the molecule is in an undefined state,because the charge of the two dots is almost the same. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-2 -1.5 -1 -0.5  0  0.5  1  1.5  2MUT aggregated charge [e]delta Z  [nm]Driver logic state 1, shifted of deltaZ,deltaYDot 1Dot 2delta Y -0.20 nmdelta Y 0 nmdelta Y +0.25 nmFig. 4. Dot charges as function of the driver shifting along the vertical axes(delta Z) for three different distances from the molecule (delta Y).Regarding the driver misalignment with the respect tothe two dots of the MUT (delta X), the trend of the dotcharges is reported in Fig. 5 for the ideal distance between theMUT and the driver (delta Y = 0). In this case, we reportedsimultaneously both the driver logic state at 1 and at 0, in orderto check immediately the MUT bistability for a specific driverposition. In particular, for a given delta X value it is possibleto check if the positive charge inside the MUT moves betweenthe two dots when the driver changes its state from 1 to 0 andthe difference between them is great enough to be considereda valid state. Figure 5 reveals that in some cases of drivermisalignment the interaction with the MUT is still preserved.The results we obtained in this work are fundamental toaddress the fabrication of a molecular QCA wire on gold. Inparticular, the fault tolerance analysis discussed here highlightsthe feasibility and functionality of the MQCA wire also inpresence of several defects we faced during our experimental 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-0.4 -0.2 0 0.2 0.4MUT aggregated charge [e]delta X  [nm]Driver shifted of delta X @ deltaY 0 nm Driver@ logic 0Driver@ logic 1Dot1Dot2Fig. 5. Dot charges as function of the driver misalignment with the respectof the two dots (delta X) for a fixed molecule-molecule distance (delta Y=0)processes. These results are promising for the developmentof molecular QCA circuits and, at the same time, give animportant feedback to improve the technological process.REFERENCES[1] Semiconductor Industry Association, International Technology Roadmapof Semiconductors, 2010 Update, Emerging research Device (ERD).[Online]. Available: http://public.itrs.net, 2010.[2] C.S. Lent, P.D. Tougaw, W. Porod, and G. Bernstein, Quantum cellularautomata, Nanotechnology, vol. 4, pp. 49-57, 1994[3] C. S. Lent, B. Isaksen and M. Lieberman, Molecular Quantum-DotCellular Automata, J. Am. Chem. Soc., vol. 125, pp. 1056-1063, 2003[4] C.S. Lent and B. Isaksen, Clocked molecular quantum-dot cellularautomata, Electron Devices, IEEE Transactions on, vol.50, no.9, pp.1890-1896, Sept. 2003[5] L. Zoli, PhD Dissertation, Universita´di Bologna, 2010.[6] V. Arima, M. Iurlo, L. Zoli, S. Kumar, M. Piacenza, F. Della Sala, F.Matino, G. Maruccio, R. Rinaldi, F. Paolucci, M. Marcaccio, P.G.Cozziand A.P. Bramanti, Toward quantum-dot cellular automata units:thiolated-carbazole linked bis-ferrocenes, Nanoscale, vol.4, pp.813-823,2012.[7] A. Pulimeno, M. Graziano, C. Abrardi, D. Demarchi, and G. Piccinini,Molecular QCA: A write-in system based on electric fields, IEEENanoelectronics Conference (INEC), vol., no., pp.1-2, June 2011[8] A. Pulimeno, M. Graziano, A. Sanginario, V. Cauda, D. Demarchi, andG. Piccinini, Bis-ferrocene molecular QCA wire: ab-initio simulations offabrication driven fault tolerance, IEEE Transactions on Nanotechnol-ogy, in press.

English

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Fault Tolerance Analysis of a Bis-Ferrocene QCA Wire

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