Download Novel Molecular Non-Volatile Memory: Application of Redox

Review
Novel Molecular Non-Volatile Memory:
Application of Redox-Active Molecules
Hao Zhu 1,2 and Qiliang Li 2, *
Received: 1 December 2015; Accepted: 21 December 2015; Published: 26 December 2015
Academic Editor: Hung-Yu Wang
1
2
*
State Key Laboratory of Application-Specific Integrated Circuits and System, School of Microelectronics,
Fudan University, Shanghai 200433, China; [email protected]
Department of Electrical and Computer Engineering, George Mason University, Fairfax, VA 22030, USA
Correspondence: [email protected]; Tel.: +1-703-993-1596; Fax: +1-703-993-1601
Abstract: This review briefly describes the development of molecular electronics in the application
of non-volatile memory. Molecules, especially redox-active molecules, have become interesting due
to their intrinsic redox behavior, which provides an excellent basis for low-power, high-density
and high-reliability non-volatile memory applications. Recently, solid-state non-volatile memory
devices based on redox-active molecules have been reported, exhibiting fast speed, low operation
voltage, excellent endurance and multi-bit storage, outperforming the conventional floating-gate
flash memory. Such high performance molecular memory will lead to promising on-chip memory
and future portable/wearable electronics applications.
Keywords: molecular electronics; redox-active molecules; non-volatile memory; high density;
high endurance
1. Introduction
The continuous complementary metal-oxide-semiconductor (CMOS) scaling is reaching the
fundamental limits imposed by heat dissipation and short-channel effects. This will finally stop the
increase of integration density and the ability of metal-oxide-semiconductor field-effect transistors
(MOSFET) predicted by Moore’s law [1–4]. Huge demand has been thus created for high-performance
semiconductor devices for future applications in memory and emerging portable/wearable electronic
devices. Molecular technology has been aggressively pursued for its potential impact in nano-scale
electronics since the early 1970s due to the inherent scalability and intrinsic properties [5–7]. Molecular
memory, among all of the new emerging candidates in recent years, is considered promising,
particularly for the aim of reducing the size per cell and enhancing the memory speed, density
and reliability [8–10]. Molecular electronic devices are typically fabricated by forming a self-assembled
monolayer (SAM) or multi-layer on different surfaces with very inexpensive and simple processing
methods. Moreover, molecular memory works by the controlling of fewer electrons at the molecule
scale and, therefore, has potential for low-power and ultra-high-density memory applications with a
lower fabrication cost. Redox-active molecules have become attractive to replace the floating gate in
conventional flash memory, due to their inherent and reliable redox behaviors. Typically, applying
an oxidation voltage will cause electron loss in redox molecules; reversely, under a reduction voltage,
electrons will be driven back to the molecules. Due to the oxidation and reduction of the redox
centers, these redox molecules can exhibit distinct charged or discharged states, which can be deemed
as logic on and off states, at different voltages with very fast write and erase speeds. It has been
demonstrated that redox-active molecules attached to Si structures are stable and can endure more than
1012 program/erase (P/E) cycles [11]. This excellent performance is naturally derived from the intrinsic
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properties of the redox behavior in molecules. These advantageous properties of redox molecules
make them very promising candidates for future applications in on-chip non-volatile memory, such as
SRAM and dynamic random-access memory (DRAM). In addition, due to the scalability of molecules
and the naturally-derived multiple redox states for robust charge storage, the molecular memory
density can be much higher than the conventional memory devices.
Great efforts have been made to integrate molecules as the active component for future electronic
devices. In this paper, we review the development of molecular electronics for non-volatile memory
applications with a focus on redox-active molecules, meanwhile presenting further strategies to extend
their applications in future low-power and ultra-high-density non-volatile memory applications.
2. Molecular Logic Switching Devices
Two fundamentally different approaches to molecular electronics are graphically termed “top-down”
and “bottom-up”. Instead of top-down, which includes making nano-scale structures by machining
and etching techniques, molecular electronics rely on the “bottom-up” approach, by taking
advantage of molecule self-assembly. Bottom-up refers to building organic or inorganic structures by
atom-by-atom or molecule-by-molecule techniques. In the past few decades, research on molecular
electronics has been focusing more and more on the combination of top-down device fabrication
(mainly lithography) with bottom-up molecule self-assembly.
The crossbar molecular circuit is one of the earliest logic forms of molecular memory arrays. The
switching element is a metal/molecule/metal sandwich junction, wherein the molecules are located at
the cross-section of two nanoscale metal wires (Figure 1). The early demonstration of such junction
molecular switching devices utilized molecules consisting of two mechanically-interlocked rings [12–15].
The molecular monolayers were deposited as a Langmuir–Blodgett film. The mechanical motion of
these molecules is an activated redox process, and two stable and electrochemically-switchable states
of the molecules form the logic on and off states. As shown in Figure 1, the molecular switching
devices can exhibit a ~102 on/off current ratio, but with limited endurance cycles. Based on such
switching devices, an 8 ˆ 8 crossbar has been built by the Hewlett-Packard research group [16]. This
approach has the advantage of architectural simplicity and the potential of high density via fabrication
of highly dense nanowires. However, this approach has two major disadvantages, including a high
rate of defective switching elements and the difficulty in controlling the metal/molecule interface.
Even though a defect-tolerant architecture had been established earlier, the metal/molecule problems
in such junction switching devices and crossbar circuits are still to be addressed [17]. In later
publications, it was reported that the electron transport phenomena in the metal/molecule/metal
junction were molecule independent; instead, they were dominated by electrode reactions with
molecules, and the intrinsic spacing of the energy levels might be modified [18–20].
Another architecture utilizing metal/molecule/metal junction is the so-called nanocell molecular
circuit, proposed by researchers at Rice University and Yale University [21–24]. A nanocell is a
two-dimensional network of self-assembled metallic particles connected by molecules that show
reprogrammable negative differential resistance (illustrated in Figure 2) [21,22]. The nanocell is
surrounded by a small number of lithographically-defined metal lines that provide input and output
leads. The active component for a nanocell is also a metal/molecule/metal switch, and one pathway
may include more than one molecular switch [25–27]. The molecule pathways can be learned first by
developing appropriate computation algorithms; then, the whole cell can be programmed to perform
a particular function. Such a nanocell molecular circuit uses similar molecular switches as the crossbar
molecular circuit, but the circuit architecture is quite different. Compared to the crossbar approach, the
nanocell approach has the advantages of large-scale circuit fabrication and integration. However, the
disadvantage is that it relies on complex programming algorithms. Meanwhile, both the nanocell and
crossbar approaches may have the same difficulty in controlling the metal/molecule interface.
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Figure 1. (Top) Atomic force microscopy image of a nanoscale cross-point molecular switching device
Figure
1. (Top)
(Top)Atomic
Atomic
force
microscopy
image
a nanoscale
cross-point
molecular
switching
Figure 1.
force
microscopy
image
of a of
nanoscale
cross-point
molecular
switching
device
with [2] a rotaxane monolayer sandwiched between two metal line electrodes; (Bottom) device
device
with
[2] a rotaxane
monolayer
sandwiched
between
two
metalline
lineelectrodes;
electrodes; (Bottom)
(Bottom) device
with [2]
a rotaxane
monolayer
sandwiched
between
two
metal
device
resistance read at 0.2 V after multiple switching cycles. Reproduced with permission from [14], © AIP
resistance
resistance read
read at
at 0.2
0.2 V
V after
after multiple
multiple switching
switching cycles.
cycles. Reproduced
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with permission
permission from
from [14],
[14], ©
© AIP
AIP
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Publishing
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Figure
2. SEM
SEM image
image of
of aananocell
nanocell after
afterassembly
assembly of
ofthe
theAu
Aunanowires
nanowires and
andmononitro
mononitro oligo(phenylene
oligo(phenylene
Figure 2.
Figure 2. SEM
image of aTop:
nanocell
after
assembly
of the Au
nanowires
and mononitro
oligo(phenylene
ethynylene)
compound.
five
pairs
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leads
across
the
nanocell;
Bottom:
higher
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ethynylene) compound. Top: five pairs of leads across the nanocell; Bottom: higher magnification of
of
ethynylene)
compound.
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the nanocell; Bottom:
higher
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of
the
is is
affixed
viavia
thethe
derivative
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the central
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portion of
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an attachedAu
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which
affixed
derivative
the centraloligo(phenylene
portion of the nanocell
with an
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nanowire,
which
is[22],
affixed
via the derivative
mononitro
ethynylene).
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with
permission
from
©
American
Chemical
of mononitro oligo(phenylene ethynylene). Reproduced with permission from [22], © American
of mononitro
oligo(phenylene ethynylene). Reproduced with permission from [22], © American
Society,
2003.
Chemical
Society, 2003.
Chemical Society, 2003.
In addition
been
tested
as
addition to
tothe
themetal/molecule/metal
metal/molecule/metal switching
switching devices,
devices,molecules
moleculeshave
havealso
also
been
tested
In addition to the metal/molecule/metal switching devices, molecules have also been tested
the
channel
material
in ainstandard
MOSFET
structure
[28–31].
As As
shown
in Figure
3, by
the
as the
channel
material
a standard
MOSFET
structure
[28–31].
shown
in Figure
3, replacing
by replacing
as the channel material in a standard MOSFET structure [28–31]. As shown in Figure 3, by replacing
high-k
dielectric
withwith
an ultra-thin
SAM
of an
active
layer
in ain
field-effect
transistor
(FET),
the
the high-k
dielectric
an ultra-thin
SAM
oforganic
an organic
active
layer
a field-effect
transistor
(FET),
the high-k dielectric with an ultra-thin SAM of an organic active layer in a field-effect transistor (FET),
Infineon
researchers
havehave
reported
an organic
field-effect
transistor
(OFET)(OFET)
with low
operation
voltage
the Infineon
researchers
reported
an organic
field-effect
transistor
with
low operation
the Infineon researchers have reported an organic field-effect transistor (OFET) with low operation
voltage and low gate leakage current [31]. The demonstrated OFETs operate with a supply voltage
voltage and low gate leakage current [31]. The demonstrated OFETs operate with a supply voltage
3
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and
low
gate
leakage
current
The lower
demonstrated
operate with
supply
less than
of
less
than
2 V,
yet have
gate [31].
currents
than thatOFETs
of the traditional
Si aFET
withvoltage
the SiOof
2 dielectric.
2
V,
yet
have
gate
currents
lower
than
that
of
the
traditional
Si
FET
with
the
SiO
dielectric.
However,
However, the switching speed of the OFET is still not comparable to Si transistors,
and the limited
2
the switching
of theasOFET
comparable
to Si for
transistors,
the limitedAnother
device
device
lifetimespeed
remains
one isofstill
thenot
major
bottlenecks
further and
applications.
lifetime remains
as one
of theismajor
bottlenecks
applications.and
Another
disadvantage
of
disadvantage
of such
OFETs
the poor
stabilityfor
in further
a high temperature
volatile
environment.
such
OFETs
is
the
poor
stability
in
a
high
temperature
and
volatile
environment.
Thus,
the
application
Thus, the application of organic molecules in advanced silicon CMOS technology is still under
of organicand
molecules
advanced
siliconefforts.
CMOS technology is still under question and requires further
question
requiresinfurther
research
research efforts.
Figure 3.
Structureand
and electrical
electrical characteristics
characteristics ofof aa molecular
molecular FET.
FET. (a)
(a) Structure
Structure of
of
Figure
3. Structure
(18-phenoxyoctadecyl)trichlorosilane
(b) structure
of the organic
pentacene;
(18-phenoxyoctadecyl)trichlorosilane(PhO-OTS);
(PhO-OTS);
(b) structure
of the semiconductor
organic semiconductor
(c)
cross-section
of a pentacene
organic field-effect
transistor transistor
(OFET); (d)
output
transfer
pentacene;
(c) cross-section
of a pentacene
organic field-effect
(OFET);
(d)and
output
and
characteristics,
with
subthreshold
region
showing
a
subthreshold
swing
of
100
mV
per
decade
transfer characteristics, with subthreshold region showing a subthreshold swing of 100 mV [31].
per
Reproduced
with permission
[31], © Nature
Publishing
2004. Group, 2004.
decade
[31]. Reproduced
withfrom
permission
from [31],
© NatureGroup,
Publishing
Redox-ActiveMolecule-Based
Molecule-BasedNon-Volatile
Non-Volatile Memory
Memory
3. Redox-Active
Appropriate modifications
thethe
molecular
structures
and switching
elements
have been
designed
Appropriate
modificationsof of
molecular
structures
and switching
elements
have
been
to
change
the
switching
kinetics
and
to
enhance
the
performance
for
different
memory
applications.
designed to change the switching kinetics and to enhance the performance for different memory
Different device
platforms
have
been engineered
interface with
the molecules,
such
as dielectrics,
applications.
Different
device
platforms
have beentoengineered
to interface
with the
molecules,
such
oxides,
nanowires,
carbon
nanotubes,
andnanotubes,
so forth. Aand
well-known
method
to achieve
a memory
effect
as
dielectrics,
oxides,
nanowires,
carbon
so forth. A
well-known
method
to achieve
a
is by employing
redox-active
Theelements.
redox mechanism
electrons
being
transferred
memory
effect is by
employingelements.
redox-active
The redoxinvolves
mechanism
involves
electrons
being
from one element
to another,
that a donor
gives element
up an electron
(oxidation)
to an
accepting
transferred
from one
elementsuch
to another,
suchelement
that a donor
gives up
an electron
(oxidation)
element
(reduction).
This(reduction).
process usually
external
stimulus
for redoxstimulus
reactions,
as
to
an accepting
element
This requires
process an
usually
requires
an external
forsuch
redox
an electric such
field or
in temperature.
Suchinredox-active
molecule-based
memory
devices can
reactions,
as change
an electric
field or change
temperature.
Such redox-active
molecule-based
show fastdevices
speed, low
operation
voltagelow
andoperation
high reliability,
the intrinsic
redox
of the
memory
can show
fast speed,
voltagedue
andtohigh
reliability,
duebehavior
to the intrinsic
molecules.
Therefore,
it
is
very
attractive
for
DRAM
and
flash
memory
device
applications.
redox behavior of the molecules. Therefore, it is very attractive for DRAM and flash memory
The
research group at North Carolina State University proposed hybrid CMOS/molecular
device
applications.
memory
which
redox-active
charge-storage
incorporated
into Si
The devices,
research in
group
at the
North
Carolina State
Universitymolecules
proposedwere
hybrid
CMOS/molecular
structuresdevices,
to generate
a new class
of electronic memory
devices molecules
[32–36]. These
molecules,
memory
in which
the redox-active
charge-storage
wereredox-active
incorporated
into Si
which
canto
begenerate
designed
to self-assemble
on surfaces
asdevices
monolayers,
exhibit
states at
structures
a new
class of electronic
memory
[32–36].
Thesecharge-storage
redox-active molecules,
discretecan
voltage
levels. to
This
approach can
transition
from
CMOS technology
which
be designed
self-assemble
on provide
surfaces aassmooth
monolayers,
exhibit
charge-storage
states to
at
discrete voltage levels. This approach can provide a smooth transition from CMOS technology to
4
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molecular electronics
electronicstechnology
technology
by integrating
theinfrastructure
vast infrastructure
of developed
CMOS
by integrating
the vast
of developed
CMOS technology
technology
with
inherentproperties.
molecular properties.
with inherent
molecular
Figure
Figure 4.
4. Structure
Structure of
of redox-active
redox-active molecules
molecules (a)
(a) Fc-BzOH
Fc-BzOH and
and (b)
(b) Por-BzOH
Por-BzOH for
for memory
memory applications;
applications;
(c)
(c) schematic
schematic of
of an
an electrolyte-molecule-Si
electrolyte-molecule-Si capacitor
capacitor with
with aa simplified
simplified equivalent
equivalent circuit.
circuit. Reproduced
Reproduced
with permission from [36], © John Wiley
Wiley and
and Sons,
Sons, 2004.
2004.
This approach focuses on developing hybrid molecular non-volatile memory with redox-active
This approach focuses on developing hybrid molecular non-volatile memory with redox-active
molecules as the active charge-storage medium. The redox-active molecule used in such molecular
molecules as the active charge-storage medium. The redox-active molecule used in such molecular
memory usually consists of a redox-active component, a linkage component and surface attachment
memory usually consists of a redox-active component, a linkage component and surface attachment
groups. Figure 4 shows two examples of such molecules with different redox-active components.
groups. Figure 4 shows two examples of such molecules with different redox-active components.
Physically, the redox-active component acts as the charge-storage center, with both the linkage and
Physically, the redox-active component acts as the charge-storage center, with both the linkage and the
the surface group acting as the insulator. From this standpoint, different redox-active component can
surface group acting as the insulator. From this standpoint, different redox-active component can be
be designed or synthesized for multiple redox states, thus for complex and high memory density.
designed or synthesized for multiple redox states, thus for complex and high memory density. The
The linkage component can be engineered to tune the memory retention properties, while the surface
linkage component can be engineered to tune the memory retention properties, while the surface
attachment groups can be specifically designed for attachment on different surfaces via covalent
attachment groups can be specifically designed for attachment on different surfaces via covalent
bonds. The redox component, such as the Fe in 4-ferrocenylbenzyl alcohol (Fc-BzOH) and Zn in
bonds. The redox component, such as the Fe in 4-ferrocenylbenzyl alcohol (Fc-BzOH) and Zn in
5-(4-hydroxymethylphenyl)-10,15,20-trimesitylporphinatozinc(II) (Por-BzOH) (shown in Figure 4), is
5-(4-hydroxymethylphenyl)-10,15,20-trimesitylporphinatozinc(II) (Por-BzOH) (shown in Figure 4),
the charge-storage medium, which can be in neutral and positively-charged states through losing
is the charge-storage medium, which can be in neutral and positively-charged states through losing
electrons. The molecular components surrounding the redox center act as the barrier against charge
electrons. The molecular components surrounding the redox center act as the barrier against charge loss.
loss. The electrons tunnel through the barrier during oxidation and reduction processes. The
The electrons tunnel through the barrier during oxidation and reduction processes. The electrolyte/
electrolyte/molecule interface acts as another barrier between the molecule and top gate electrode,
molecule interface acts as another barrier between the molecule and top gate electrode, which blocks
which blocks the electron tunnel into the molecule from the electrolyte to reduce an oxidized
the electron tunnel into the molecule from the electrolyte to reduce an oxidized molecule. The positive
molecule. The positive charges in the molecule can be reduced at a lower gate potential, wherein an
charges in the molecule can be reduced at a lower gate potential, wherein an electron tunnels back to
electron tunnels back to the molecule from the Si substrate.
the molecule from the Si substrate.
The SAM attachment of redox molecules to Si, SiO2 or other surfaces was typically carried out
The SAM attachment of redox molecules to Si, SiO2 or other surfaces was typically carried out
by using chemical solution deposition [35–37]. The attachment is characterized by using cyclic
by using chemical solution deposition [35–37]. The attachment is characterized by using cyclic
voltammetry (CyV) measurement, a current-voltage measurement at various voltage scan rates.
voltammetry (CyV) measurement, a current-voltage measurement at various voltage scan rates.
Figure 5a,b shows the CyV curves of electrolyte-molecule-oxide-Si (EMOS) structures with the
Figure 5a,b shows the CyV curves of electrolyte-molecule-oxide-Si (EMOS) structures with the Fc-BzOH
Fc-BzOH and Por-BzOH redox molecules attached to SiO2 for charge storage, respectively [35,37].
and Por-BzOH redox molecules attached to SiO2 for charge storage, respectively [35,37]. The CyV
The CyV curves of both molecular structures demonstrate oxidation and reduction peaks. Fc-BzOH
curves of both molecular structures demonstrate oxidation and reduction peaks. Fc-BzOH shows
shows one pair of redox peaks, because the redox center Fe only demonstrates a neutral state and one
one pair of redox peaks, because the redox center Fe only demonstrates a neutral state and one
positively-charged state. However, the Zn redox center in Por-BzOH exhibits a neutral state and two
positively-charged state. However, the Zn redox center in Por-BzOH exhibits a neutral state and two
(mono- or di-) positively-charged states; therefore, two pairs of redox peaks were observed.
(mono- or di-) positively-charged states; therefore, two pairs of redox peaks were observed.
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Figure 5.5. Cyclic
Figure
Cyclic voltammetry
voltammetry of
of electrolyte-molecule-oxide-Si
electrolyte-molecule-oxide-Si (EMOS)
(EMOS)structures
structureswith
with(a)
(a) the
the
2
at
different
voltage
scan
rates;
Fc-BzOH
and
(b)
the
Por-BzOH
redox
molecule
attached
to
SiO
Fc-BzOH and (b) the Por-BzOH redox molecule attached to SiO2 at different voltage scan rates;
capacitance-voltage (C-V)
(C-V) and
and conductance-voltage
conductance-voltage (G-V)
capacitance-voltage
(G-V)curves
curvesof
of(c)
(c)Fc-BzOH
Fc-BzOHand
and(d)
(d)Por-BzOH
Por-BzOH
EMOS
capacitors
at
100
Hz.
Reproduced
with
permission
from
[35,37],
©
AIP
Publishing
LLC, 2003,
EMOS capacitors at 100 Hz. Reproduced with permission from [35,37], © AIP Publishing
LLC,
2004.2004.
2003,
The capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics also exhibited
The capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics also exhibited clear
clear capacitance and conductance peaks associated with trapping and detrapping of charge in the
capacitance and conductance peaks associated with trapping and detrapping of charge in the molecules.
molecules. Similarly, multiple peaks were observed with the Por-BzOH EMOS capacitor due to its
Similarly, multiple peaks were observed with the Por-BzOH EMOS capacitor due to its two charged
two charged states. Interestingly, the two charged states within molecules, such as Por-BzOH, can be
states.
Interestingly, the two charged states within molecules, such as Por-BzOH, can be employed
employed for a multi-bit storage application to further enhance the memory density. Despite using a
for
a
multi-bit
application
further
enhance
the memory
Despite
usingstrategy
a redox
redox moleculestorage
exhibiting
multiple to
charged
states,
an alternative
and density.
perhaps more
efficient
molecule
multiple
states,
an alternative
and perhaps
more efficient
strategy
to achieveexhibiting
higher memory
bitscharged
is afforded
by simply
mixing different
redox-active
molecules
whoseto
achieve
higher
memory
bits
is
afforded
by
simply
mixing
different
redox-active
molecules
whose
potentials are well separated. Fc-BzOH and Por-BzOH SAMs have been mixed on Si, and the mixed
potentials
are
well
separated.
Fc-BzOH
and
Por-BzOH
SAMs
have
been
mixed
on
Si,
and
the
mixed
SAMs exhibit well-defined redox states [36]. The mixed SAM approach is very attractive owing
to
SAMs
exhibit
well-defined
redox
states
[36].
The
mixed
SAM
approach
is
very
attractive
owing
to
the
the fact that this approach is synthetically far simpler than preparing a single molecule that exhibits
fact
that
approach
is synthetically
simpler
than
a single molecule
that distinct
exhibitsthan
three
three
orthis
more
redox states.
In addition,far
the
potential
of preparing
the redox charged
states is more
or
more
redox
states.
In
addition,
the
potential
of
the
redox
charged
states
is
more
distinct
than
that of a single molecular redox center with multiple charged states. However, the disadvantagethat
of
of
a single
molecular
redox
centerofwith
multiple
states.
However,
the disadvantage
this
this
method
is that the
density
a given
peakcharged
goes down
[38].
Nevertheless,
this mixed of
SAM
method
is that
the density
of afor
given
peak goes
down [38].
Nevertheless,
this mixed SAM approach
approach
still paves
the way
constructing
multi-bit
memory
storage devices.
still paves the way for constructing multi-bit memory storage devices.
4. Solid-State Molecular Memory
4. Solid-State Molecular Memory
The conventional polysilicon floating-gate flash memory has benefited the semiconductor
The conventional
polysilicon
floating-gate
flash memory
has benefited
the semiconductor
memory
memory
technology for
decades, due
to its scalability,
compatibility
with CMOS
process, and so
forth.
technology
to its from
scalability,
compatibility
CMOS
process,
and
so forth. Itoxide
relies
It relies on for
hotdecades,
electron due
injection
the channel
into thewith
floating
gate
through
a tunneling
on
hot
electron
injection
from
the
channel
into
the
floating
gate
through
a
tunneling
oxide
layer. By attaching redox-active molecules to Si as the floating gate in the CMOS structure, onelayer.
can
By
attaching
redox-active
to Sireduce
as the the
floating
in the CMOS
structure, one
further
further
enhance
the cell molecules
density and
cell gate
variations.
The combination
of can
top-down
enhance
the cell
and reduce
theself-assembly
cell variations.
The combination
of top-down
lithography
and
lithography
anddensity
bottom-up
molecule
processes
renders a uniform
charge
density, and
the monodisperse nature of the molecular orbitals leading to distinct energy levels can enable precise
6
Appl. Sci. 2016, 6, 7
7 of 15
bottom-up molecule self-assembly processes renders a uniform charge density, and the monodisperse
nature of the molecular orbitals leading to distinct energy levels can enable precise controlling of
charged states [39]. Moreover, with a solid-state insulating barrier deposited on both sides of the
molecules, the possibility of orbital hybridization from the gate will be further lessened. Such an
integration of redox-active molecules in a solid-state memory provides an efficient platform to study
the molecules and devices with microelectronic characterization metrologies, which is unique in
comparison with the liquid electrolyte-based electrochemical method.
The self-assembly process of the redox-active molecules to form a solid-state molecular memory
enables not only high-quality attachment of molecules to semiconductor surfaces, but also uniform
and high-density fabrication of low-cost devices and circuits. The disadvantages of molecular flash
memory mainly lie in its reliability in volatile environments. Some redox-active molecules are instable
or even do not function at temperatures higher than 200 ˝ C due to evaporation or redox center reaction.
Under volatile environments with oxygen or water, the molecule’s bond might be broken, leading
to poor redox behavior. As a result, some specific conditions and environments need to be taken
into account when integrating such redox molecules in CMOS devices and circuits. Future molecular
technology requires advancement in both molecule properties and device integration processes.
The researchers from North Carolina State University reported a study on preventing contact
metal penetration into the molecular layer in the metal-insulator-molecule-metal structure [40].
Similar work demonstrating that redox properties of molecules can be preserved after thin film
encapsulation, such as atomic layer deposition (ALD), initiated the investigation on high-performance
solid-state molecular memory [41]. The Cornell University research group reported a study on a
metal-oxide-semiconductor (MOS) structure with molecules encapsulated in a high-k dielectric and
functioning as the charge storage medium [42,43]. As shown in Figure 6, molecules with different
redox centers, which can show one and two charged states, have been studied for multi-bit memory.
With the electrostatics determined by the alignment between the highest occupied or the lowest
unoccupied molecular orbital (HOMO or LUMO, respectively) energy levels and the charge neutrality
level of the dielectric, the asymmetric charge injection behavior in flat-band voltage shift (∆VFB )
vs. programming voltage measurement (Figure 7) is due to the Fermi-level pinning between the
molecules and the high-k dielectric [44–46]. The three programmable molecular orbital states of the
5-(4-Carboxyphenyl)-10,15,20,-triphenyl-porphyrin-Co(II) (CoP) molecule have been experimentally
observed and are attractive for low-variation multi-bit memory applications. However, the charge
retention of this capacitor molecular memory is still far below the modern flash memory requirements.
The engineering by the same group on the tunnel layer has greatly enhanced the electron retention
and programming performance by forming an organic-inorganic tunnel barrier [43]. Nevertheless,
the endurance of this molecular memory (~104 cycles) is still quite limited; further engineering
and improvement on the molecular orbitals and device structure are needed to achieve higher
memory performance.
Recently, the NIST researchers have reported a similar capacitor-structure molecular non-volatile
memory, but with enhanced charge retention and excellent endurance [47]. The molecular memory
uses an α-ferrocenylethanol redox molecule, which has a very simple structure and one redox center.
As shown in Figure 8, with the molecules attached to the SiO2 tunnel oxide and encapsulated in
the ALD Al2 O3 dielectric, the solid-state molecular memory exhibited excellent memory behavior,
such as a sufficient memory window, good retention and endurance. Even though only ~15% of the
molecules were effectively involved in the redox processes after ALD, effective memory can be realized
with sufficient charge density. These characteristics are attributed to the intrinsic redox behavior of
the ferrocene molecule and the effective protection from the presence of the gate dielectric covering
the molecule SAM. The molecular memory showed excellent endurance properties, with negligible
memory window degradation after 109 P/E cycles, which is about 10,000-times better than that of
the conventional floating gate memory (1 ˆ 105 P/E cycles). However, the operation voltage of such
molecular memory is still high, and the operation speed needs to be improved. Further study of the
Appl. Sci. 2016, 6, 7
8 of 15
molecular memory depends on the engineering of redox-active molecules, such as molecules with
multiple
redox
centers
and a proper linker for multi-bit memory, and the device structure engineering
Appl.
Appl. Sci.
Sci. 2016,
2016, 6,
6, 77
to optimize the dielectric stack thickness and improve the molecule/dielectric interface. Such an
Such anwill
exploration
will be a breakthrough
significant breakthrough
in the
for charge-storage
non-volatile
exploration
be a significant
in the quest
forquest
charge-storage
non-volatile
memory
memory
andthe
will
enable the application
of flash-like
devices
foron-chip
future on-chip
memory
applications.
and will
enable
application
of flash-like
devices for
future
memory
applications.
Figure
6.
of
(MOS)
capacitor
with
Figure
6. Schematics
of the metal-oxide-semiconductor
(MOS) capacitor
structure
with (a)structure
ferrocenecarboxylic
Figure
6. Schematics
Schematics
of the
the metal-oxide-semiconductor
metal-oxide-semiconductor
(MOS)
capacitor
structure
with
(a)
ferrocenecarboxylic
acid
(FcCOOH)
and
(b)
CoP
SAM
attached
to
Si
structures.
Reproduced
with
(a) ferrocenecarboxylic
acid
(FcCOOH)
andto
(b)SiCoP
SAM attached
to Si structures.
Reproducedfrom
with [42],
acid (FcCOOH)
and (b) CoP
SAM
attached
structures.
Reproduced
with permission
permission
permission
from [42],
[42], ©
© IEEE,
IEEE, 2011.
2011.
© IEEE,
2011. from
Figure
7. High-frequency
C-V
and
as
ofprogramming
programming
voltage
at K
10and
K and
Figure
7.
C-V
and
ΔV
function
voltage
at
room
FBFB
Figure
7. High-frequency
High-frequency
C-V
and∆V
ΔV
FB as
as aaa function
function of
of
programming
voltage
at 10
10
K
and
roomroom
temperature
for the
with
(a,b)
FcCOOH
CoPmolecules.
molecules.
Reproduced
with
permission
temperature
for
the
with
(a,b)
FcCOOH
and
(c,d)
Reproduced
with
permission
temperature
for device
the device
device
with
(a,b)
FcCOOHand
and (c,d)
(c,d) CoP
CoP
molecules.
Reproduced
with
permission
©
2011.
from from
[42],
©
IEEE,
2011.
from [42],
[42],
© IEEE,
IEEE,
2011.
8
Appl. Sci. 2016, 6, 7
Appl. Sci. 2016, 6, 7
9 of 15
Appl. Sci. 2016, 6, 7
Figure
8.8. (a)
(a)Molecule
Molecule
structure
of α-ferrocenylethanol
and a schematic
of the
Figure
structure
of α-ferrocenylethanol
and a schematic
of the Metal-Al
2O3Metal-Al
-Ferrocene2 O3 Ferrocene-Oxide-Silicon
(MAFOS)
capacitor
memory
structure;
(b)
∆V
of
the
molecular
memory
FB
of
the
molecular
memory
and
control
Oxide-Silicon
(MAFOS)
capacitor
memory
structure;
(b)
ΔV
Figure 8. (a) Molecule structure of α-ferrocenylethanol and a schematic of FB
the Metal-Al2O3-Ferroceneand
controlassamples
as aofcapacitor
function
of program/erase
voltage;
(c) charge
retention
at room
samples
a function
program/erase
(P/-E)
voltage;
(c)FBcharge
at
room
temperature;
of
the retention
molecular
memory
and control
Oxide-Silicon
(MAFOS)
memory
structure;
(b)(P/-E)
ΔV
(d)
deviceasendurance
differentwith
P/Epulse
widths.
Reproduced
with
permission
from
[47],
© AIP
samples
a function
of program/erase
(P/-E)
voltage;
(c) charge
retention
at room
temperature;
temperature;
(d)
devicewith
endurance
different
P/Epulse
widths.
Reproduced
with
permission
(d)[47],
device
endurance
different
P/Epulse widths. Reproduced with permission from [47], © AIP
Publishing
LLC,
2013.with LLC,
from
© AIP
Publishing
2013.
Publishing LLC, 2013.
Nanowire/Nanotube-Based Molecular
Molecular Memory
5. 5.
Nanowire/Nanotube-Based
Memory
5. Nanowire/Nanotube-Based Molecular Memory
Semiconductor nanowire and nanotube MOSFETs have been regarded as the building blocks for
Semiconductor nanowire and nanotube MOSFETs have been regarded as the building blocks for
Semiconductor
nanowire
and nanotube
have
been regarded
the building
for
nanoelectronics
devices
and circuits
[48–50].MOSFETs
Compared
to planar
devices as
based
on bulk blocks
materials,
nanoelectronics devices and circuits [48–50]. Compared to planar devices based on bulk materials, the
nanoelectronics
devices
and
circuits
[48–50].
Compared
to
planar
devices
based
on
bulk
materials,
the nanowires have smaller channels and a larger surface-to-volume ratio. Thus, they require less
nanowires have smaller channels and a larger surface-to-volume ratio. Thus, they require less stored
the nanowires
smaller
and a larger
surface-to-volume
ratio.
Thus, they
require less
stored
charges have
to induce
thechannels
same memory
window
for the memory
application.
Moreover,
the
charges
induce to
theinduce
same memory
window
for the memory
Moreover,Moreover,
the nanowire
storedtocharges
the samecan
memory
for theapplication.
memory
application.
the or
nanowire
or nanotube
channel
enablewindow
a gate-surrounding
structure,
allowing excellent
nanotube
channel
can enable
a gate-surrounding
allowing excellent electrostatic
gate
control.
nanowire
orgate
nanotube
can enable
astructure,
gate-surrounding
allowing
excellent
electrostatic
control.channel
Integrating
redox-active
molecules withstructure,
semiconductor
nanowire
or
Integrating
redox-active
molecules
with
semiconductor
nanowire
or
nanotube
FETs
for
solid-state
flash
electrostatic
gate
control.
Integrating
redox-active
molecules
with
semiconductor
nanowire
or
nanotube FETs for solid-state flash memory will not only improve the memory performance by
memory
will
not
only
improve
the
memory
performance
by utilizing
the
properties
nanotube
FETs
for solid-state
flash
memory
will not molecules,
only
improve
memory
performance
by of
utilizing
the
advantageous
properties
of redox-active
but the
alsoadvantageous
will provide
an
efficient
redox-active
molecules,
but
also
will
provide
an
efficient
platform
to
study
the
redox-active
molecules
utilizing
the
advantageous
properties
of
redox-active
molecules,
but
also
will
provide
an
efficient
platform to study the redox-active molecules with microelectronics characterization metrologies.
with
microelectronics
metrologies.
platform
to study thecharacterization
redox-active molecules
with microelectronics characterization metrologies.
Figure 9. Nanowire-based non-volatile memory, with the nanowire surface functionalized with
Figure 9. Nanowire-based
non-volatile
memory,
nanowire by
surface
functionalized
with
redox-active
molecules. Logic
on and memory,
off stateswith
are the
represented
different
charged states.
Figure
9. Nanowire-based
non-volatile
with
the
nanowire surface
functionalized
with
redox-active
molecules.
Logic
on
and
off
states
are
represented
by
different
charged
states.
Reproduced
with permission
© American
Chemicalby
Society,
2002.
redox-active
molecules.
Logic onfrom
and[51],
off states
are represented
different
charged states. Reproduced
Reproduced with permission from [51], © American Chemical Society, 2002.
with permission from [51], © American Chemical Society, 2002.
9
9
Appl. Sci. 2016, 6, 7
Appl. Sci. 2016, 6, 7
10 of 15
In 2002, the researchers from Harvard University reported a molecular non-volatile memory
based on a back-gated nanowire FET with the nanowire surface functionalized with cobalt
In 2002, the researchers from Harvard University reported a molecular non-volatile memory based
phthalocyanine (CoPc) redox-active molecules (Figure 9) [51]. By modulating the channel
on a back-gated nanowire FET with the nanowire surface functionalized with cobalt phthalocyanine
conductance with back gate voltage, reversible switching between on and off states has been achieved,
(CoPc) redox-active molecules (Figure 9) [51]. By modulating the channel conductance with back
with the on/off ratio exceeding 104. The retention (>20 min) and endurance (>100 cycles) of the
gate voltage, reversible switching between on and off states has been achieved, with the on/off
molecular memory device have outperformed the earlier junction molecular switching devices.
ratio exceeding 104 . The retention (>20 min) and endurance (>100 cycles) of the molecular memory
Integrated arrays of independently-addressable molecular nanowire memory have also been
device have outperformed the earlier junction molecular switching devices. Integrated arrays
characterized, which demonstrated another advantage of implementing nanowire structure memory
of independently-addressable molecular nanowire memory have also been characterized, which
in programmable logic arrays. The memory performance has been further improved with molecule
demonstrated another advantage of implementing nanowire structure memory in programmable logic
and device engineering. In 2004, the University of Southern California research groups reported a
arrays. The memory performance has been further improved with molecule and device engineering.
back-gate nanowire FET-based molecular memory, which utilized In2O3 nanowires and a family of
In 2004, the University of Southern California research groups reported a back-gate nanowire FET-based
bis(terpyridine)-Fe(II) molecules for charge storage (Figure 10) [52,53]. It was demonstrated that
molecular memory, which utilized In2 O3 nanowires and a family of bis(terpyridine)-Fe(II) molecules
bis(terpyridine)-Fe(II) with a larger or more complex linkage component showed better retention,
for charge storage (Figure 10) [52,53]. It was demonstrated that bis(terpyridine)-Fe(II) with a larger or
due to the increase in the tunneling barrier. In addition, by applying gate voltage pulses with different
more complex linkage component showed better retention, due to the increase in the tunneling barrier.
amplitudes, multilevels in memory cells can be illustrated by altering the population of the
In addition, by applying gate voltage pulses with different amplitudes, multilevels in memory cells
reduced/oxidized molecules [53]. However, such multilevel memory realization depends on the gate
can be illustrated by altering the population of the reduced/oxidized molecules [53]. However, such
voltage applied to oxidize/reduce a portion of molecules in SAM. Further cell scaling or increasing
multilevel memory realization depends on the gate voltage applied to oxidize/reduce a portion of
the number of memory levels will be extremely difficult, as the complicated top-down device
molecules in SAM. Further cell scaling or increasing the number of memory levels will be extremely
fabrication and device structure will inevitably lead to significant device variation and a blurring of
difficult, as the complicated top-down device fabrication and device structure will inevitably lead to
the multiple storage states. In addition, the demonstrated device performance is still far below the
significant device variation and a blurring of the multiple storage states. In addition, the demonstrated
standards for on-chip memory applications. This is partly because of the back gate device structure,
device performance is still far below the standards for on-chip memory applications. This is partly
unclean steps in the nanowire FET device fabrication process and, most importantly, the absence of
because of the back gate device structure, unclean steps in the nanowire FET device fabrication process
protection for redox-active molecules.
and, most importantly, the absence of protection for redox-active molecules.
Figure 10. Comparison of designs and operations between (a) conventional silicon flash memory and
(b)
multilevel
nonvolatile
datadata
storage;
(c) comparison
of multilevels
in a two-bit
(b) molecular
molecularmemory
memoryforfor
multilevel
nonvolatile
storage;
(c) comparison
of multilevels
in a
silicon
memory
molecular
memory; (d)
molecular
Fe2+ -terpyridine
compound.
two-bitflash
silicon
flash and
memory
and molecular
memory;
(d) structure
molecularof structure
of Fe2+-terpyridine
Reproduced
with permission
[53,54],from
© AIP
Publishing
2004,2007.
compound. Reproduced
withfrom
permission
[53,54],
© AIPLLC,
Publishing
LLC, 2004,2007.
10
Appl. Sci. 2016, 6, 7
Appl. Sci. 2016, 6, 7
11 of 15
Recently,
the NIST
NIST researchers
researchers have
have extended
extended their
their work
work on
on the
the capacitor
capacitor memory
memory cell
cell to
to aa novel
Recently, the
novel
top-gated
nanowire
molecular
flash
memory
based
on
Si
nanowire
FETs
[47,55].
The
memory
cell
top-gated nanowire molecular flash memory based on Si nanowire FETs [47,55]. The memory cell
consists
consists of
of aa self-aligned
self-aligned top-gated
top-gated Si
Si nanowire
nanowire FET
FET with
with α-ferrocenylethanol
α-ferrocenylethanol redox-active
redox-active molecules
molecules
attached
to
a
Si
nanowire
surface
(Figure
11).
The
molecules
are
effectively
protected
by engineered
attached to a Si nanowire surface (Figure 11). The molecules are effectively protected by engineered
top
top
dielectrics.
injected
charges
are mainly
located
the redox
centers
of molecules,
with
gategate
dielectrics.
The The
injected
charges
are mainly
located
in theinredox
centers
of molecules,
with trace
trace
amount
the molecule/Al
2O3 interface traps. The molecular flash memory cell exhibited
amount
storedstored
in thein
molecule/Al
2 O3 interface traps. The molecular flash memory cell exhibited fast
9
fast
P/E
speed
and
excellent
P/E
endurance
cycles).
These
characteristics
are attributed
to
P/E speed and excellent P/E endurance (>109(>10
P/EP/E
cycles).
These
characteristics
are attributed
to both
both
the intrinsic
charging
properties
of redox-active
molecules
the protection
of the molecules
the intrinsic
charging
properties
of redox-active
molecules
and the and
protection
of the molecules
obtained
obtained
by
the
self-alignment
fabrication
enabling
clean
solid-molecule
and
dielectric
by the self-alignment fabrication enabling clean solid-molecule and dielectric interfaces. interfaces.
However,
However,
similar
to theircapacitor
previousmolecular
capacitor molecular
memory,
of the were
molecules
were
similar to their
previous
memory, only
~20% only
of the~20%
molecules
effectively
effectively
the redox
process.
Further
research
efforts are
requiredthe
to redox
increase
the redox
involved ininvolved
the redoxinprocess.
Further
research
efforts
are required
to increase
efficiency
so
efficiency
so
as
to
lower
the
operation
voltage
and
improve
the
operation
speed.
The
memory
as to lower the operation voltage and improve the operation speed. The memory density can bedensity
further
can
be further
enhanced
by using redox
molecules
with
multiple
redox
centers.
Such aconcept
multi-bit
enhanced
by using
redox molecules
with multiple
redox
centers.
Such
a multi-bit
memory
is
memory
concept
is
more
reasonable
and
feasible
than
that
by
just
modulating
the
voltage
level,
as
more reasonable and feasible than that by just modulating the voltage level, as the physically discrete
the
physically
charged states
of the precise
molecule
can enableofprecise
controlling
of the charged states.
charged
statesdiscrete
of the molecule
can enable
controlling
the charged
states.
Figure 11.
11. (Left)
(Left) TEM
TEM image
image of
of the
the cross-section
cross-section of
of aa molecular
molecular flash
flash memory
memory device;
device; (Right)
(Right) discrete
discrete
Figure
charging behavior
flash memory and the endurance properties. Reproduced with
2 molecular
2 molecular flash memory and the endurance properties. Reproduced
charging
behaviorofofthe
theRu
Ru
permission
from
[55],
©
American
Chemical
Society,
2015. 2015.
with permission from [55], © American Chemical
Society,
In addition to the traditional redox-active molecules, novel molecules and clusters have been
synthesized and implemented in molecular memory.
memory. In 2014, Busche et al. reported
reported aa study on
molecular nanowire flash memory using core-shell polyoxometalate (POM) molecules for charge
storage (Figure 12) [56]. The
The POM molecule
molecule functions
functions as the switching component
component in the memory
associated with the oxidation
of
selenite
template
at
the
cluster
core
and
the reduction of the metal
oxidation
template
oxide cluster cage. The
The unique
unique oxidation
oxidation process
process of
of the
the selenite
selenite template
template at
at the
the cluster
cluster core was
observed only after the initial excitation and cannot
cannot be
be recreated
recreated with consecutive
consecutive operations, leading
to a new
new type
typeofof“write-once-erase”
“write-once-erase”memory.
memory.
The
flash
memory
with
a 4-nm
Si nanowire
channel
The
flash
memory
with
a 4-nm
Si nanowire
channel
and
and
for charge
storage
has shown
excellent
retention
properties
no charge
POMPOM
for charge
storage
has shown
excellent
retention
properties
with with
no charge
store store
decaydecay
after
after
hours.
the increasing
state molecules,
of POM molecules,
theratio
on/off
can
336 h.336
With
the With
increasing
oxidationoxidation
state of POM
the on/off
canratio
reach
asreach
high as
11
11
high[56].
as 10
Even
thevoltage
operation
and the
time of memory
such molecular
10
Even[56].
though
thethough
operation
and voltage
the switching
timeswitching
of such molecular
are still
memory
arefurther
still quite
large, further
engineering
to shorten
from
gate
to the
quite
large,
engineering
to shorten
the distance
from the distance
control gate
tothe
thecontrol
nanowire
channel
nanowire channel and to reduce the number of POMs can be expected to effectively lower the voltage
and the write time into the sub-picosecond region.
11
Appl. Sci. 2016, 6, 7
12 of 15
and to reduce the number of POMs can be expected to effectively lower the voltage and the write time
region.
Appl.
6, 7
into Sci.
the2016,
sub-picosecond
Image of
of the
thenanowire
nanowire flash
flash memory
memory device
device and
and the
the drain
drain current
current behavior
behavior with an applied
applied
Figure 12. Image
Cross-sectional
image
(left)
of the
device
and the
top
view
voltage to
tothe
thecontrol
controlgate.
gate.(a)(a)
Cross-sectional
image
(left)
of memory
the memory
device
and
the
top(right)
view
of
a
device
with
a
5-nm
Si
nanowire
channel
and
side
control
gate;
measurements
of
the
logarithmic
(right) of a device with a 5-nm Si nanowire channel and side control gate; measurements of the
(b) and linear
drain
current
versuscurrent
gate voltage
0.5voltage
V source-drain
before
deposition
of
logarithmic
(b)(c)and
linear
(c) drain
versus at
gate
at 0.5 V voltage:
source-drain
voltage:
before
the polyoxometalates
(POMs) (green(POMs)
dashes),
after dashes),
the deposition
of deposition
the POMs (orange
dashes),
after
deposition
of the polyoxometalates
(green
after the
of the POMs
(orange
a ´20 V after
pulsea (blue
after
a 120
pulse
(redVline).
with permission
from [56],
dashes),
−20 Vline)
pulseand
(blue
line)
andVafter
a 120
pulseReproduced
(red line). Reproduced
with permission
©
American
Chemical
Society,
2014.
from [56], © American Chemical Society, 2014.
As illustrated by the above experimental work, the performance
performance of molecular
molecular flash memory
depends
on
not
only
the
redox
behavior
of
the
molecules,
but
also
the
preparation
of the transistor
depends
platform and the integration of the molecules in the memory structure. While the memory
memory endurance
is mainly determined
by
the
mechanism
of
the
charge
storage,
further
improving
the memory
determined
performance, such as the memory
memory density,
density, speed and power, requires optimized engineering on the
transistor platform structure
structure and
and an
an enhanced
enhanced molecule/semiconductor
molecule/semiconductor interface. For example, a high
quality
tunneling
oxide
layer
in
a
molecular
flash
memory with
with less
less molecule/dielectric
molecule/dielectric interface
quality tunneling oxide layer in a molecular flash memory
interface traps
traps
can
significantly improve
memory retention
retention and
and lower
lower the
the operation
operation voltage.
voltage. The
The semiconductor
semiconductor
can significantly
improve the
the memory
nanowire
have been
been shown
shown to
to be
nanowire FETs
FETs have
be promising
promising building
building blocks
blocks to
to create
create new
new functional
functional molecular
molecular
memory
devices.
A
vast
array
of
other
device
platforms,
such
as
Si
surfaces,
metal
oxide
and
carbon
memory devices. A vast array of other device platforms, such as Si surfaces, metal oxide
and
nanotubes,
can becan
implemented
for molecular
memory
if an
appropriate
carbon nanotubes,
be implemented
for molecular
memory
if an
appropriatemodification
modificationcan
can be
be
designed
to
interface
the
molecules
with
such
platforms.
Therefore,
the
integration
of
redox-active
designed to interface the molecules with such platforms. Therefore, the integration of redox-active
molecules
molecular flash
memory is
very attractive,
attractive, because
molecules as
as the
the active
active component
component in
in aa molecular
flash memory
is very
because it
it will
will
leverage
theadvantages
advantagesafforded
afforded
redox-active
molecules
theinfrastructure
vast infrastructure
of
leverage the
byby
thethe
redox-active
molecules
with with
the vast
of current
current
semiconductor
technology.
semiconductor technology.
Conclusions
6. Conclusions
In the past few years, the performance
performance of
of molecular
molecular memory
memory has
has been
been pushed
pushed to
to new
new frontiers.
frontiers.
molecules in Langmuir–Blodgett
Langmuir–Blodgett film, more and more
more new
new
Developed from the mechanical motion of molecules
families of molecules
molecules have been synthesized,
synthesized, and
and the redox properties have been investigated
investigated for
families
memory applications.
applications. Up
Up to
to now,
now, redox-active
redox-active molecules
molecules have
have already shown their potential and
memory
future
low-voltage,
high-density
and high-reliability
non-volatile
memory.
advantageous properties
propertiesforfor
future
low-voltage,
high-density
and high-reliability
non-volatile
The current
main
barriers
the CMOS
compatibility
and the issues
introduced
with the molecular
memory.
The
current
mainare
barriers
are the
CMOS compatibility
and the
issues introduced
with the
integration integration
with semiconductor
devices. The realization
of future
molecular
memory
applications
still
molecular
with semiconductor
devices. The
realization
of future
molecular
memory
requires a combination
of empirical
fabrication
characterization
with
sophisticated
application
applications
still requires
a combination
of and
empirical
fabrication
and
characterization
with
of rational designs
for particular
molecular
devices.
Uponelectronics
this, theredevices.
will no doubt
be a
sophisticated
application
of rational
designselectronics
for particular
molecular
Upon this,
significant
breakthrough
in micro-/nano-electronics
at replacing the SRAM,
the CPU
there
will no
doubt be a significant
breakthrough inaiming
micro-/nano-electronics
aimingboosting
at replacing
the
ability and
creating
portable/wearable
electronic
devices.
SRAM,
boosting
theenhanced
CPU ability
and creating enhanced
portable/wearable
electronic devices.
Acknowledgments: The work described here was supported by US National Science Foundation (NSF) Grant
ECCS-0846649 and Virginia Microelectronics Consortium Research Grant.
Conflicts of Interest: The authors declare no conflict of interest.
Appl. Sci. 2016, 6, 7
13 of 15
Acknowledgments: The work described here was supported by US National Science Foundation (NSF) Grant
ECCS-0846649 and Virginia Microelectronics Consortium Research Grant.
Conflicts of Interest: The authors declare no conflict of interest.
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