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Mechanochemical preparation of piezoelectric nanomaterials: BN, MoS2 and WS2 2D materials and their glycine-cocrystals

03 Feb 2020-Vol. 5, Iss: 2, pp 331-335

AbstractDifferent 2D-layered materials of transition metal dichalcogenides (TMDCs) such as boron nitride (BN) or molybdenum disulphide (MoS2) have been theorised to have piezoelectric behaviour. Still, the procedures to obtain these nanomaterials, with the right quality and quantity to observe the piezoelectric performance, are enormously expensive, halting their possible applications. Here, we show the mechanochemical exfoliation of 2D nanomaterials (FLG, BN, MoS2 and WS2) with glycine. We have also successfully synthesised the cocrystals for these nanomaterials, which makes it possible to enhance their piezoelectric responses.

Topics: Exfoliation joint (57%), Boron nitride (52%), Nanomaterials (50%)

Summary (1 min read)

Jump to: [Introduction] and [Conclusions]

Introduction

  • Starting with graphene, 2D nanomaterials have grown to include insulator (boron nitride, BN), semiconductors (molybdenum disulphide, MoS2) and metals (Niobium diselenide, NbSe2).
  • The structure of glycine cocrystals has been investigated, showing the presence of γ and β-glycine.
  • This could correspond to the appearance of new crystal forms and it also gives information on the quality of the exfoliated nanomaterials (Fig. S10).
  • The piezoelectricity behaviour is better in the cocrystal form than in the exfoliated material or with unpolarized PZT powders (table 1, Fig. S12 and Fig S13) or other organic piezoelectric materials.

Conclusions

  • The process includes the preparation of glycine- 2D cocrystals in which the proportion of different polymorphisms of glycine is readily changed.
  • These powder samples, with different 2D materials, can be used in the development of matrices with piezoelectric character.
  • These materials could be imbedded on paints or inks to cover large surfaces, either in mobile devices, tablets, keyboard, with improved piezoelectric properties.

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COMMUNICATION
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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Mechanochemical preparation of piezoelectric nanomaterials: BN,
MoS
2
and WS
2
2D materials and their glycine-cocrystals
Viviana Jehová González,
a
Antonio M. Rodríguez,*
a
Ismael Payo,
b
Ester Vázquez*
a,c
Different 2D-layered materials of transition metal dichalcogenides
(TMDCs) such as boron nitride (BN) or molybdenum disulphide
(MoS
2
) have been theorised to have piezoelectric behaviour. Still,
the procedures to obtain these nanomaterials, with the right
quality and quantity to observe the piezoelectric performance, are
enormously expensive, halting its possible applications. Here, we
show the mechanochemical exfoliation of 2D nanomaterials (FLG,
BN, MoS
2
and WS
2
) with glycine. We have also successfully
synthesised the cocrystals for these nanomaterials, which makes it
possible to enhance their piezoelectric responses.
Piezoelectric materials have a unique property that converts
mechanical energy into electrical energy or viceversa
1
. Barium
titanate is the first piezoelectric ceramic ever discovered, but
the ceramic lead zirconate titanate, also known as PZT, is the
most commonly used material for piezoelectric harvesting.
2
Nevertheless, the extremely fragile nature of PZT ceramic and
the incorporation of lead create issues such as the reliability,
durability, and safety of this material for long-term sustainable
operation.
2D materials and the possibility to modulate their composition
in a well-controlled manner offer a platform that allows the
creation of different heterostructures for a large variety of
applications. Starting with graphene, 2D nanomaterials have
grown to include insulator (boron nitride, BN), semiconductors
(molybdenum disulphide, MoS
2
) and metals (Niobium
diselenide, NbSe
2
).
3
Together with other different properties,
the theoretical piezoelectricity of single-atomic layers of boron
nitride (BN), molybdenum disulfide (MoS
2
) and tungsten
disulfide (WS
2
) as a function of strain-induced lattice distortion
and ionic charge polarisation has been studied.
4, 5
The future
perspective of these nanomaterials have been covered in the
literature.
6
Experimentally, some applications of this
nanomaterial behaviour have been explored in energy
conversion,
7
voltage generators,
8
pressure sensors,
9
nonlinear
energy harvesters,
10
and transducers.
11
The methodologies
currently used in the production of these nanomaterials for the
nano-electromechanical applications are mainly based on
chemical vapour deposition (CVD). This technique presents
some problems, such as the high cost and necessity to deposit
on other materials, which can lead to compatibility issues.
Additionally, in real-world applications, the environmental
impact of producing any device should always be considered
beforehand, and one fundamental problem is to scale up
experiments in a safe, secure and efficient way. In that sense,
mechanochemical exfoliation of 2D materials has gained
increasing importance in the last years.
12-15
These protocols
have many advantages over their liquid-phase counterparts,
including processes with shorter reaction times, higher product
yields and the elimination of (harmful) organic solvents, which
make the approach more sustainable and cheaper. Some
examples have seen molecules such as sucrose, urea and boric
acid used as exfoliating agents.
16-18
Nowadays, there are no
examples of the application of TMDCs nanomaterials in
piezoelectric paint, coatings or adhesive matrices which could
be easily applied to heterogeneous surfaces paving the way for
applications such as sensors,
19
or power sensors
20
and
nanosystems for harvesting energy applications.
21, 22
On the other hand, in the past 60 years, piezoelectricity has
been confirmed in a variety of biological materials, such as
fibrous proteins collagen,
23
elastin,
24
bone
25
(calcified collagen),
wood,
26
and some viruses
27
exhibit relatively modest
piezoelectricity (0.110 pm V
−1
). Classical piezoelectric
principles have also been applied to similar uniaxially
orientated, bioactive polymers, such as poly (L-lactic acid)
(PLLA), poly (γ-benzyl glutamate) (PBG), and cellulose.
28
The
only non-chiral amino acid, glycine, has been known to
crystallize in three distinct polymorphs (α)-alpha,
29
)-beta,
30
and (γ)-gamma glycine
31
under ambient conditions.
32
The
a.
Instituto Regional de Investigación Científica Aplicada (IRICA), UCLM, 13071
Ciudad Real, Spain.
b.
Escuela de Ingeniería Industrial y Aeroespacial de Toledo, UCLM, Avenida Carlos
III s/n, Real Fábrica de Armas, 45071, Toledo, Spain.
c.
Facultad de Ciencias y Tecnologías Químicas, UCLM, Avda. Camilo José Cela S/N,
13071, Ciudad Real, Spain.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x

COMMUNICATION Journal Name
2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
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crystallization of α-glycine occurs in the centrosymmetric space
group P2
1
/c, which precludes piezoelectricity. On the other
hand, β-glycine and γ-glycine belong to the non-
centrosymmetric space groups P21 and P32, respectively, and
so should exhibit a non-zero piezoelectric response. A modest
‘effective' shear and longitudinal piezoelectricity have been
measured for β-glycine (6 pm V
−1
) and γ-glycine (10 pm V
−1
),
respectively,
33, 34
using piezo response force microscopy
(PFM).
35
In previous work, we have investigated the exfoliation
procedures of graphite to graphene using ball milling
techniques in the presence of carbohydrates.
36
We could also
prepare glucose-graphene cocrystals as biocompatible systems.
In this work, we have explored the exfoliation of 2D
nanomaterials using the amino acid glycine. In a second step,
the formation of glycine-nanomaterials cocrystals has proven to
enhance the piezoelectric properties of the exfoliated material.
The relative ease of production of these materials through our
mechano-chemical process would significantly impact its
presence in future applications. In this study, we proposed a
mechanochemical exfoliation of TMDCs and other 2D
nanomaterials, such as BN and FLG, and the study of its intrinsic
piezoelectricity. Furthermore, our objective aims to integrate
the TMDCs nanomaterials in supramolecular organic matrices,
such as cocrystals that would enhance their piezoelectricity.
Based on our previous experience on mechanochemical
exfoliation of graphite, we performed the ball-milling treatment
in solvent-free conditions adding glycine as the exfoliant agent
and graphite in a 250 mL stainless-steel grinding bowl with 15
stainless steel balls (2 cm diameter each) at a 250 rpm. The
detail experimental procedure is collected in the SI. Since, no
precipitate was observed in the resulting dispersions, they were
entirely lyophilised after the dialysis. The best experimental
conditions for obtaining graphene materials of two different
sizes and the yields are represented in table S1.
Fig. S1 displays the Raman spectra of FLG1 and FLG2, showing
the different characteristic bands present in carbon
nanomaterials (D, G and 2D).
37, 38
It is possible to observe the
I
D
/I
G
value between the different peaks in sample FLG1 is 0.39
in comparison with FLG2, 1.63. This data correlates with the
minor size of FLG2 flakes (Table S1, Fig. S2) which shows a direct
relation with the time of mechanochemical treatment.
Thermogravimetrical analysis (TGA) of these materials is
collected in Fig. S3. Our analysis showed a minor presence of
nitrogen attached on the graphene layer with a minor presence
of oxygen and organic groups on the surface of graphene (2%
loss in TGA). We can draw similar conclusions regarding the TGA
loss for both FLG1 and FLG2 nanomaterials as in our previous
works.
36
Based on these good results, the high-quality exfoliation with
very high yields and the smooth, sustainable and low-cost
procedure, we decided to extrapolate these experimental
procedures to the exfoliation of other 2D-layered materials
such as BN, MoS
2
and WS
2
. The experimental conditions for the
2D nanomaterial exfoliation are collected in Table S1. Powder
X-Ray Diffraction (PXRD) of the exfoliated materials and the raw
nanomaterials are all shown in Fig. S4. In all cases, the x-ray
diffraction patterns show a clear decrease in the number of
counts on the 002-diffraction pattern. For the example of BN,
which has the lowest reduction, it is known that intensity ratio
of (BN
raw
)
002
/ (BN
exfo
)
002
of approximately 2.5 already indicates
thin BN layers and much weaker stacking at the c-direction in
the exfoliated sample.
39, 40
Fig. S5 shows the Raman spectra of the MoS
2
system, although
both WS
2
and MoS
2
have similar patterns. Both nanomaterials
possess two primary Raman modes, one in-plane mode of Mo
or W-S bond (E
2g
) another out-of-plane mode (A
1g
) at around
380 and 405 cm
-1
(MoS
2
), and 350 and 415 cm
-1
(WS
2
).
41
It is
possible to observe a blueshift and a redshift in A
1g
mode for
MoS
2
and WS
2
respectively, which corresponds to a decrease in
the number of layers (Table S2). According to the diagrams of
Terrones et al. for WS
2
nanomaterials,
42
we have a relation of
I
E2g
/I
A1g
of 0.69 which corresponds to a value of 3 layers for our
WS
2
exfoliated nanomaterial. With respect to MoS
2
,
43
according
to the distance between the bands
𝐸
"#
$
and
%𝐴
$#
,
the average
number of layers is around 3. Finally, BN exhibits a characteristic
Raman peak for E2g phonon mode (B-N vibration mode) around
1365 cm
-1
44
which is analogous to E2g mode (G band) in
graphene. Moreover, a slight blue shift in the E2g peak is
consistent with the exfoliation of BN.
45
TEM images (Fig. 1)
show the exfoliated dichalcogenide with the corresponding
distribution of lateral size in table S1. As shown in Fig. S6, the
Figure 1. TEM images and distributions of sizes for the different 2D nanomaterials samples.
Figure 2. Powder X-ray diffraction results for glycine and BNglycine cocrystals.

Journal Name COMMUNICATION
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TGA curves of exfoliated nanomaterials show a reduced weight
loss compared with those of the pristine 2D nanomaterial
because of the small residue of exfoliant agent. Also, a wide
scan XPS spectra has been included in the SI to rule out the
presence of other impurities (Fig. S7 and Table S3). The atomic
content (in %) corresponding to C, N, H and O also correlates
with the small quantities of exfoliant agent. Raw and exfoliated
materials are not expected to contain C and O. However around
10% of the undesired C and O content probably arises from CO
and CO
2
species in air, adsorbed on the substrates.
46
Nevertheless, further analysis of the samples has shown that
the sample BN
exfo
has a residue amount of glycine around 20%,
which also corresponds to our TGA analysis (in Fig. S6).
These results are similar to those observed in the literature.
47
Once demonstrated the exfoliation of 2D nanomaterials, we
studied the formation of glycine cocrystals following a similar
procedure of lyophilization (SI). The PXRD study for all the
different materials indicated that cocrystal structures differed
wildly from the original nanomaterials or the initial glycine
crystal structure (Fig. 2 for the BN). The appearance of some
new peaks (between 20 and 65º) correspond to the different
reflections of
a
,
b
, and
g
-glycine phases in the cocrystal sample
and other new peaks in that same region, which do not
correspond to any raw material. Those new peaks can be
attributed to new cocrystal structures. Similar results are
observed for other nanomaterials (Fig. S8). It seems that the
presence of the nanomaterial in dispersion together with the
crystallization of water while freezing, “pressed out” glycine
forcing the appearance of different polymorphisms. This is a
process known in the literature,
48, 49
and it correlates well with
our understanding on the important interactions between
water molecules, exfoliating agents and 2D materials.
50
The TGA
for the 2D nanomaterial cocrystals shows a similar loss to
glycine, which might be due to the high content of such
molecule. The 3wt% of difference between glycine and the
cocrystal measurement, corresponds to the presence of the
nanomaterial in the cocrystal structure.
The structure of glycine
cocrystals has been investigated, showing the presence of γ and
β-glycine. Commercial glycine was similarly grinded as
benchmark sample, resulting in β-glycine majority and with
similar piezoelectric response to initial glycine. A comparison of
the powder X-ray diffraction results of these samples can be
found in Fig. S9. Further study of the Raman spectra of 2D
nanomaterial glycine cocrystals pointed to modifications on the
vibrational mode frequencies of the intermolecular and
intramolecular bonds in the samples. This could correspond to
the appearance of new crystal forms and it also gives
information on the quality of the exfoliated nanomaterials (Fig.
S10).
Finally, preliminary studies of the piezoelectricity of the
exfoliated nanomaterials and the glycine cocrystal forms were
performed. Fig. 3 shows the experimental setup used for
dynamic testing of the piezoelectricity. Results were amplified
with an electronic circuit as shown in Fig. S11. The
nanomaterials were placed in a simple system, sandwiched
between electrodes of area 1 cm
2
, under a force of 10 N. This
experimental setup mimics those described in the literature for
the experimental corroboration of the piezoelectricity of
different materials in powder form.
51
Table 1. Comparison of piezoelectric response raw and exfoliated 2D
nanomaterials and its cocrystals. PZT has been used as a model
piezoelectric material.
Sample
Piezoelectric response
(mV·N
-1
)
PZT
16
Polyvinylidene fluoride (PVDF)
48
Glycine (Gly)
12
Gly
grinded
15
Gly
lyophilised
36
BN
raw
8
BN
exfo
48
BN- Gly cocrystal
64
BN
exfo
+ Gly-mix
37
WS
2 raw
--
a
WS
2 exfo
--
b
WS
2
Gly cocrystal
88
WS
2 exfo
+ Gly-mix
60
Graphite
--
a
FLG
exfo
--
a
FLG- Gly cocrystal
95
FLG
exfo
+ Gly-mix
60
MoS
2 raw
--
b
MoS
2 exfo
--
b
MoS
2
- Gly
cocrystal
150
MoS
2 exfo
+ Gly-mix
78
a
These nanomaterials can’t be properly measured because its relatively
high conductivity, it short-circuited the electronic.
b
Given the
semiconductor behaviour of MoS
2
, the materials could be working as
super-capacitor in the measurement. Attached the figures in the
supplementary material (SI).
The piezoelectricity behaviour is better in the cocrystal form
than in the exfoliated material or with unpolarized PZT powders
(table 1, Fig. S12 and Fig S13) or other organic piezoelectric
materials. We obtained similar results for all nanomaterials in
their cocrystal form, with a maximum open-circuit voltage of
150 mV·N
-1
for the MoS
2
-Gly
cocrystal
. Similar results could be
Figure 3. a) Experimental set up for piezoelectricity measurements. b) Layer of
cocrystals on a square copper electrode (10 mm × 10 mm), insulated with paper.
c). Manual compression of a
2D nanomaterial
crystal layer. d) Piezoelectric
response of exfoliated
BN
nanomaterials. e) Measured piezoelectric response
with exfoliated
BN cocrystals.

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4 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx
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observed with the MoS
2
and WS
2
at the same range of induced
strain (Table 1). For comparison purposes, we also mixed
thoroughly the samples of the exfoliated samples and the
glycine cocrystal separately (Table 1 samples: 2D nanomaterial
+
Gly
mix
), but it showed less piezoelectric response.
Also, both BN and WS
2
cocrystals showed outstanding
responsiveness under lower ranges of forces (around 1N) and
produced good recovery cycles and maintained in time (Fig.
S14). The remarkable piezoelectric character of these
nanomaterials has all been measured without polarisation,
while the standard procedure uses polarized materials for this
sort of measurements.
Conclusions
We describe an easy and scalable method to enhance the
piezoelectric responses of 2D nanomaterials. The process
includes the preparation of glycine- 2D cocrystals in which the
proportion of different polymorphisms of glycine is readily
changed. These powder samples, with different 2D materials,
can be used in the development of matrices with piezoelectric
character. These materials could be imbedded on paints or inks
to cover large surfaces, either in mobile devices, tablets,
keyboard, with improved piezoelectric properties. Future uses
in sensors, power sensors or in harvesting energy applications
can be predicted.
Conflicts of interest
There are no conflicts to declare.
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Journal ArticleDOI

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08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

30,199 citations


Journal Article
Abstract: Ferroelectrics are multifunctional materials that reversibly change their polarization under an electric field. Recently, the search for new ferroelectrics has focused on organic and bio-organic materials, where polarization switching is used to record/retrieve information in the form of ferroelectric domains. This progress has opened a new avenue for data storage, molecular recognition, and new self-assembly routes. Crystalline glycine is the simplest amino acid and is widely used by living organisms to build proteins. Here, it is reported for the first time that {gamma}-glycine, which has been known to be piezoelectric since 1954, is also a ferroelectric, as evidenced by local electromechanical measurements and by the existence of as-grown and switchable ferroelectric domains in microcrystals grown from the solution. The experimental results are rationalized by molecular simulations that establish that the polarization vector in {gamma}-glycine can be switched on the nanoscale level, opening a pathway to novel classes of bioelectronic logic and memory devices.

85 citations


Journal ArticleDOI
Abstract: Two-dimensional transition metal dichalcogenides (2D TMDs) have attracted much attention in the field of optoelectronics due to their tunable bandgaps, strong interaction with light and tremendous capability for developing diverse van der Waals heterostructures (vdWHs) with other materials. Molybdenum disulfide (MoS2) atomic layers which exhibit high carrier mobility and optical transparency are very suitable for developing ultra-broadband photodetectors to be used from surveillance and healthcare to optical communication. This review provides a brief introduction to TMD-based photodetectors, exclusively focused on MoS2-based photodetectors. The current research advances show that the photoresponse of atomic layered MoS2 can be significantly improved by boosting its charge carrier mobility and incident light absorption via forming MoS2 based plasmonic nanostructures, halide perovskites–MoS2 heterostructures, 2D–0D MoS2/quantum dots (QDs) and 2D–2D MoS2 hybrid vdWHs, chemical doping, and surface functionalization of MoS2 atomic layers. By utilizing these different integration strategies, MoS2 hybrid heterostructure-based photodetectors exhibited remarkably high photoresponsivity raging from mA W−1 up to 1010 A W−1, detectivity from 107 to 1015 Jones and a photoresponse time from seconds (s) to nanoseconds (10−9 s), varying by several orders of magnitude from deep-ultraviolet (DUV) to the long-wavelength infrared (LWIR) region. The flexible photodetectors developed from MoS2-based hybrid heterostructures with graphene, carbon nanotubes (CNTs), TMDs, and ZnO are also discussed. In addition, strain-induced and self-powered MoS2 based photodetectors have also been summarized. The factors affecting the figure of merit of a very wide range of MoS2-based photodetectors have been analyzed in terms of their photoresponsivity, detectivity, response speed, and quantum efficiency along with their measurement wavelengths and incident laser power densities. Conclusions and the future direction are also outlined on the development of MoS2 and other 2D TMD-based photodetectors.

58 citations


Journal ArticleDOI
11 Aug 2020
TL;DR: The universal approaches and recent progresses in the field of lead-free piezoelectric nano-materials, initially focusing on hybrid composite materials as well as individual nanoparticles, and related energy harvesting devices are systematically elaborated.
Abstract: Current piezoelectric device systems need a significant reduction in size and weight so that electronic modules of increasing capacity and functionality can be incorporated into a great range of applications, particularly in energy device platforms. The key question for most applications is whether they can compete in the race of down-scaling and an easy integration with highly adaptable properties into various system technologies such as nano-electro-mechanical systems (NEMS). Piezoelectric NEMS have potential to offer access to a parameter space for sensing, actuating, and powering, which is inflential and intriguing. Fortunately, recent advances in modelling, synthesis, and characterization techniques are spurring unprecedented developments in a new field of piezoelectric nano-materials and devices. While the need for looking more closely at the piezoelectric nano-materials is driven by the relentless drive of miniaturization, there is an additional motivation: the piezoelectric materials, which are showing the largest electromechanical responses, are currently toxic lead (Pb)-based perovskite materials (such as the ubiquitous Pb(Zr,Ti)O3, PZT). This is important, as there is strong legislative and moral push to remove toxic lead compounds from commercial products. By far, the lack of viable alternatives has led to continuing exemptions to allow their temporary use in piezoelectric applications. However, the present exemption will expire soon, and the concurrent improvement of lead-free piezoelectric materials has led to the possibility that no new exemption will be granted. In this paper, the universal approaches and recent progresses in the field of lead-free piezoelectric nano-materials, initially focusing on hybrid composite materials as well as individual nanoparticles, and related energy harvesting devices are systematically elaborated. The paper begins with a short introduction to the properties of interest in various piezoelectric nanomaterials and a brief description of the current state-of-the-art for lead-free piezoelectric nanostructured materials. We then describe several key methodologies for the synthesis of nanostructure materials including nanoparticles, followed by the discussion on the critical current and emerging applications in detail.

20 citations


Journal ArticleDOI
Abstract: Presently, graphenic nanomaterials are being studied as candidates for wastewater pollutant removal. In this study, two graphite oxides produced from natural graphite with different grain sizes (325 and 10 mesh), their respective reduced graphene oxides and one reduced graphene oxide with nitrogen functional groups were synthesized and tested to remove a surfactant model substrate, Triton X-100, from an aqueous solution. Kinetic experiments were carried out and adjusted to pseudo-first order equation, pseudo-second order equation, Elovich, Chain-Clayton and intra-particle diffusion models. Reduced graphene oxides displayed an instantaneous adsorption due to their accessible and hydrophobic surfaces, while graphite oxides hindered the TX100 adsorption rate due to their highly superficial oxygen content. Results from the adsorption isotherms showed that the Sips model perfectly described the TX100 adsorption behavior of these materials. Higher adsorption capacities were developed with reduced graphene oxides, being maximum for the material produced from the lower graphite grain size (qe = 3.55·10−6 mol/m2), which could be explained by a higher surface area (600 m2/g), a lower amount of superficial oxygen (O/C = 0.04) and a more defected structure (ID/IG = 0.85). Additionally, three commercial high surface area graphites in the range of 100–500 m2/g were evaluated for comparison purposes. In this case, better adsorption results were obtained with a more graphitic material, HSAG100 (qe = 1.72·10−6 mol/m2). However, the best experimental results of this study were obtained using synthesized graphenic materials.

1 citations


References
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Book ChapterDOI

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01 Jan 2012

123,310 citations


Journal ArticleDOI

[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

30,199 citations


Journal Article

9,021 citations


Journal ArticleDOI
25 Jul 2013-Nature
TL;DR: With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.
Abstract: Fabrication techniques developed for graphene research allow the disassembly of many layered crystals (so-called van der Waals materials) into individual atomic planes and their reassembly into designer heterostructures, which reveal new properties and phenomena. Andre Geim and Irina Grigorieva offer a forward-looking review of the potential of layering two-dimensional materials into novel heterostructures held together by weak van der Waals interactions. Dozens of these one-atom- or one-molecule-thick crystals are known. Graphene has already been well studied but others, such as monolayers of hexagonal boron nitride, MoS2, WSe2, graphane, fluorographene, mica and silicene are attracting increasing interest. There are many other monolayers yet to be examined of course, and the possibility of combining graphene with other crystals adds even further options, offering exciting new opportunities for scientific exploration and technological innovation. Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as ‘van der Waals’) have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene’s springboard, van der Waals heterostructures should develop into a large field of their own.

6,776 citations


Journal ArticleDOI
Abstract: We review recent work on Raman spectroscopy of graphite and graphene. We focus on the origin of the D and G peaks and the second order of the D peak. The G and 2 D Raman peaks change in shape, position and relative intensity with number of graphene layers. This reflects the evolution of the electronic structure and electron–phonon interactions. We then consider the effects of doping on the Raman spectra of graphene. The Fermi energy is tuned by applying a gate-voltage. We show that this induces a stiffening of the Raman G peak for both holes and electrons doping. Thus Raman spectroscopy can be efficiently used to monitor number of layers, quality of layers, doping level and confinement.

5,730 citations


Frequently Asked Questions (1)
Q1. What are the contributions mentioned in the paper "Mechanochemical preparation of piezoelectric nanomaterials: bn, mos2 and ws2 2d materials and their glycine-cocrystals" ?

Here, the authors show the mechanochemical exfoliation of 2D nanomaterials ( FLG, BN, MoS2 and WS2 ) with glycine.