Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D

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Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures Finn Purcell-Milton, Robert McKenna, Lorcan Brennan, Conor P. Cullen, Lilian Guillemeney, Nikita V. Tepliakov, Anvar S. Baimuratov, Ivan D. Rukhlenko, Tatiana S. Perova, Georg S. Duesberg, Alexander V. Baranov, Anatoly V. Fedorov, and Yurii K. Gun'ko ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06691 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Graphic for abstract:

MoS2

D-Penicillamine

CD Active MoS2

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Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D MoS2 Nanostructures Finn Purcell-Milton,*† Robert McKenna,† Lorcan Brennan,† Conor P. Cullen,† Lilian Guillemeney, † Nikita V. Tepliakov,‡ Anvar S. Baimuratov, ‡ Ivan D. Rukhlenko,‡ Tatiana S. Perova,§ Georg S. Duesberg, † Alexander V. Baranov, ‡ Anatoly V. Fedorov, ‡Yurii K. Gun’ko*† †

School of Chemistry, University of Dublin, Trinity College, Dublin 2, Irelandal ‡

§

ITMO University, St. Petersburg, 197101, Russia

Electronic & Electrical Engineering, University of Dublin, Trinity College, Dublin 2, Ireland

Corresponding Authors *E-mails: [email protected] and [email protected] ORCID Finn Purcell-Milton: 0000-0002-3591-9477 Yurii K. Gun’ko: 0000-0002-4772-778X

KEYWORDS optically active, chiral, MoS2, 2D nanomaterials, amino acids, folding.


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ABSTRACT: Two-dimensional (2D) nanomaterials have been intensively investigated due to their interesting properties and range of potential applications. Although most research has focused on graphene, atomic layered transition metal dichalcogenides (TMDs) and particularly MoS2 have gathered much deserved attention recently. Here we report the induction of chirality into 2D chiral nanomaterials by carrying out liquid exfoliation of MoS2 in the presence of chiral ligands (cysteine and penicillamine) in water. This processing resulted in exfoliated chiral 2D MoS2 nanosheets showing strong circular dichroism (CD) signals, which were far past the onset of the original chiral ligand signals. Using theoretical modelling we demonstrated that the chiral nature of MoS2 nanosheets is related to the presence of chiral ligands causing preferential folding of the MoS2 sheets. There was an excellent match between the theoretically calculated and experimental spectra. We believe that due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer great opportunities for the development of chiroptical sensors, materials and devices for valleytronics and other potential applications. In addition, chirality plays a key role in many chemical and biological systems, with chiral molecules and materials critical for the further development of biopharmaceuticals and fine chemicals, this research therefore should have a strong impact on relevant areas of science and technology such as Nanobiotechnology, Nanomedicine and Nanotoxicology.


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Chirality is a fascinating occurrence of asymmetric shapes, giving rise to chiral molecules in the real world with two mirror-image forms, termed enantiomers, which are non-superimposable in three dimensions. Well-known examples of chiral molecules include proteins, DNA sugars, amino-acids, enzymes, and a huge range of drugs, including ibuprofen (common antiinflammatory and analgesic), L-Dopa (effective on Parkinson disease) and other commercially significant molecules, such as aspartame (artificial sweetener) and geraniol (rose-like scent in perfumes). Overall, chirality plays an important role in molecular recognition and therefore chiral compounds play a very significant role in chemistry, biology, pharmacology and medicine. Chirality has been studied in a range of nanomaterials including quantum dots,1 transition metal oxides2 and metal nanoparticles,3 since it has been envisaged to play an important role in inorganic material based nanotechnology 4 due to the range of potential applications offered by these materials including chiral sensors, catalysts and as metamaterials in advanced optical devices. 5-23 Therefore, an understanding and development of the fundamental concepts relevant to chirality in these systems is of paramount importance. Two-dimensional (2D) nanomaterials are being intensively investigated at present, though this area began with the study of graphene, atomic layered transition metal dichalcogenides (TMDs) and in particularly MoS2 have attracted much attention recently. This interest is driven by their distinctive properties (e.g. direct band gaps, conductivity, flexibility, transparency, large surface area, etc.) and a range of potential applications such as photonics, sensing, catalysis, solid lubrication, energy storage, high performance electronics and optoelectronics.24-27 MoS2 is a material of particular interest due to its semiconducting characteristics, in which its band gap energy is layer thickness dependent, and transitions from an indirect to direct band gap in fewlayered MoS2.

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In addition, MoS2 nanomaterials have been envisaged for various biological


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applications such as cell imaging, medical diagnostic tools, therapeutics and drug delivery systems.

29-32

One of the most exciting proposed applications of MoS2 and 2D TMD

nanomaterials in general is in the emerging field of valleytronics, which involves the use of the wave quantum number of an electron in a crystalline material by controlling the photon angular momentum (circular polarization state) via circular polarized light to encode data.33-38 Thus there is a significant interest and potential applications for chiral 2D nanomaterials, while little publications as of yet document these intriguing materials. Recent examples of chiral 2D nanomaterials include

chiral graphene quantum dots,

graphene nanodisk assemblies

40

39

chiroptically active plasmonic

circularly polarized light induced photooxidation and self-

assembly of CdTe nanocrystals into nanoribbons with specific helicity,

41

twisting of CdSe nanoplatelets and their self-assembly into chiral ribbons.

and ligand-induced 42

However, to the

best of our knowledge there are no reports on chiral MoS2 and TMD based 2D nanomaterials. One of the most popular methods of large-scale preparation of 2D nanomaterials, including graphene, TMDs, layered oxides, and clays

29, 43-47

is via sonication assisted liquid exfoliation.

This approach is based on the use of sonic energy to separate sheets of the material, disrupting the weak van der Waals forces coupling the sheets.48-49 Solvent choice is critical to enable this process, with best results achieved via matching the surface energy of the 2D nanomaterial to the surface tension of the solvent,50 assuming that the entropic contribution of exfoliation of rigid 2D sheets is negligible.51 In addition, surfactants can be applied to stabilise exfoliated layers, broadening the range of potential solvents these 2D nanomaterials can be produced in. 52 Here we demonstrate the preparation of chiral 2D nanomaterials by liquid exfoliation of MoS2 in the presence of appropriate chiral ligands (e.g. cysteine and penicillamine) in water by ultrasonic treatment. These chiral 2D nanomaterials have been characterised by UV-Vis


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absorption spectroscopy, circular dichroism (CD) spectroscopy, Diffuse Reflectance Infrared Fourier Transform (DRIFT), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, Transmission electron microscopy (TEM) and Scanning Transmission electron microscopy (STEM). In addition, we have also performed theoretical modelling and provided an explanation of the origin of the chiroptical activity in such 2D systems.

Results and Discussion Commercially available bulk MoS2 samples were processed by using sonication assisted exfoliation in an inert atmosphere under argon in water in the presence of chiral molecules (see the Methods section) enabling us to produce chiral ligand functionalised MoS2 flakes, which have then been investigated by various spectroscopic techniques. UV-Vis absorption spectra of the exfoliated produced MoS2 sample in the presence and absence of chiral ligands are presented in Figure 1. The spectra show two distinctive peaks, which are associated with a direct transition at the K point, giving two sharp peaks at the threshold, which are excitonic transitions, assigned as the K4 to K5 (612 nm) and K1 to K5 (673 nm). A third threshold at — 500 nm is due to a direct transition from deep in the valence band to the conduction band and are also associated with excitonic features at 405 nm and 461 nm.53 Following ligand functionalisation and a second cycle of centrifugation, a reduction in peaks intensity occurs, though the overall shape of the spectral bands is retained, as shown in the Dpenicillamine and L-cysteine spectra.


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Figure 1. UV-Vis absorption spectra of exfoliated non-functionalised MoS2 and cysteine and penicillamine functionalised MoS2.

The CD spectra of D- or L- penicillamine and D- or L- cysteine functionalised MoS2 are shown in Figure 2. There is a set of CD peaks ranging from the UV to the NIR onset. The spectra show a mirror image when functionalised with opposite ligands and displays the well-known Cotton effect,54 with peaks alternating from positive to negative CD values. It should also be noted a range of other amino acids tested as well as non-ligand functionalised MoS2 did not show any CD (see figure S1 and S2, in supporting information) The CD peaks detected for exfoliated MoS2 in the presence of cysteine and penicillamine are completely different from the CD spectra of the original penicillamine and cysteine ligands, which only show peaks in the range of 200 300 nm (see supporting information Figure S3), with cysteine functionalised MoS2 samples showing a stronger CD signal intensity. The intensity of these resulting signals was far higher than normally reported for CD active nanoparticles1-3, 17 where the origin of chirality is purely ligand induced. This indicates the potential of another possible source of the CD signal, that will be discussed in details in next sections below. The effects of concentration upon the CD signal


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was studied using penicillamine ligand and the optimal concentration value of 0.15 M was found, since concentration below this rate resulted in a lower CD response, while values above 0.15 M did not show an increase in the CD signal response. We also found that sonication of MoS2 in the presence of the chiral ligand increased the CD signal to a certain time (approximately 5-10 minutes, see supporting information, Figure S4) after which saturation of the signal occurred.

A

B

Figure 2. CD spectra of D/L Penicillamine (A) and D /L cysteine (B) exfoliated functionalized MoS2.

A range of other potential chiral ligands including amino acids (glutamic acid, alanine, methionine), amino acid derivatives (glutathione, L-cystine ethyl ester, penicillamine disulphide and cystine) and similar achiral ligands (cysteamine, 3-mercaptopropionic acid) were also tested for their ability to induce chirality upon MoS2 samples. Though we did find in most cases an effect upon the UV-Vis absorption spectroscopy, no induced CD signal was observed (See supporting information, S1 and S2). The specific reasoning for CD signals in exfoliated MoS2


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only produced using cysteine and penicillamine ligands, could be due to the following causes. Firstly, in a number of publications it has been shown that for a CD signal to be induced, not only does a ligand coordination need to occur, but that it must take place via at least two functional groups.4, 18, 55 Patterns in literature with other chalcolites also point towards a definite need for one of these functional groups to be a thiol,17, 56 enabling effective bonding to the metal centres, and may also be important to enable effective orbital overlap, via the valence electrons of the QD and HOMO of the surface ligands,57 58 in addition, a carboxylic acid is usually present as another functional group.4 Finally, proximity of the dissymmetry centre of the ligand to the nanomaterial surface has the effect of increasing the produced CD signal, which is present in the small chiral ligands of penicillamine and cysteine.59-63 Therefore, due to these results, we focused upon the penicillamine and cysteine functionalised MoS2 sheets for further characterisation, using penicillamine as a model system due to its lower rate of disulphide formation, and hence greater stability. Atomic force microscopy was used to analyse samples of penicillamine functionalised and nonligand functionalised MoS2 dried on silicon wafers. This enabled us to determine the thickness of flakes produced with results presented in Figure 3 showing the images produced and measured cross section. From these measurements, a thickness of 3-4 nm for MoS2-Pen was determined, which corresponds to 4-6 monolayers of MoS2 (0.65 - 0.7 nm each monolayer),

64-65

while we

find a thickness of 5-9 nm for non-ligand functionalised MoS2, corresponding to 7-14 monolayers of MoS2, indicating that the presence of ligands provided greater exfoliation of the MoS2 sheets. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) were also applied to study the thickness, morphology and size of MoS2 flakes produced,


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before and after ligand functionalisation. Selected results are shown in Figure 4 with further figures images present in figures S6 – S10 in supporting information. MoS2 sheets demonstrated a large polydispersity in widths ranging from 2-3 µm to 200-100 nm. No visible change was apparent from the imaging between samples before and after ligand functionalisation, though there was no distinct expectation this would be the case due to drying effects normally being the dominant factor determining the sample morphology observed in TEM and STEM imaging. This is due to the conditions of sample preparation in which the sample is drop cast upon a TEM grid and allowed to dry in ambient conditions and is widely reported to therefore demonstrate drying effects, 66-67 hence this approach cannot show the exact morphology of the flakes in solution, but only the shape adopted when dried.


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Figure 3. AFM images of MoS2 flakes on silicon substrate with accompanying side profile, with A and B representing results for MoS2 before ligand functionalisation, while C and D show the results after penicillamine ligand functionalisation.

DRIFT was used to confirm the functionalisation of the MoS2 flakes and is shown in Fig S5 in supporting information. In the DRIFT spectra of the MoS2 there were four important bands to analyse. Firstly, the band of 384 cm-1 can be used to identify the material as 2H-MoS2.68 Secondly, the band at 2532 cm-1 (or 2540) is usually associated to the SHstr. for cysteine or 2662 cm-1 for penicillamine, which if linked to the surface of the MoS2 could be partially shifted in position.69-70 Other important peaks which are relevant to the ligands present are the N-H str. asym.


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is 3066 cm-1 for D- penicillamine and 3190 cm-1 for Cys and the -CO2 str. asym. 1655 cm-1 for Dpenicillamine and 1645 cm-1, indicating the presence of the ligands. 69-70

A

B

C

D

E

F

Figure 4. TEM and STEM of MoS2 flakes produced, without ligands TEM (A,B) and STEM (C), with L-cysteine , TEM (D) and with D-penicillamine TEM (E) and STEM (F).


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Figure 5. Raman spectra of MoS2 flakes at 633 nm excitation registered for samples with no ligand (red line) and with penicillamine (blue line). Vertical dotted lines correspond to the peak positions of E12g and A1g modes for sample with no ligand, correspondent to the peak position difference of 24.5 cm-1 (this difference for sample with ligand is 23.2 cm-1 Peaks at ~ 420, 454 and 463 cm-1 are assigned to B1u, a second-order zone-edge phonon 2LA(M) and a first-order phonon peak A2u, respectively.71 Raman spectra were recorded at resonant (633 nm) excitation for samples of MoS2 deposited onto SiO2/Si substrate with a thickness of SiO2 layer of 285 cm-1. The sensitivity of Raman spectroscopy to the change of phonon frequency is widely used for determining the number of layers in a 2D crystalline structures, including graphene, MoS2 and WS2.72-74 Typically, two wellpronounced peaks, the in-plane E2g1 (at ~ 382 cm-1) and out-of-plane A1g (at ~ 407 cm-1) are observed in Raman spectrum of MoS2 at off-resonance excitation (i.e. for wavelengths below 633 nm).72-74 At resonance conditions (at 633 nm excitation correspondent to the direct band gap of 1.96 eV) the Raman spectrum of MoS2 becomes more complicated and includes a few additional modes at lower and higher frequencies (namely, 460, 572, 599 and 641 cm-1).

71

The

alteration of phonon frequencies ( ) between two modes (A1g and E2g1) is widely used ACS Paragon Plus Environment

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for determining the number of layers (or as a thickness indicator) in MoS2 nanolayers. The difference between peak positions of A1g and E2g1 modes varied between 22 (3 monolayers) and 24.5 (6 monolayers) cm-1 for MoS2 flakes deposited from solution without ligand, while for the flakes deposited from solution with ligands this difference is typically 1-2 cm-1 lower (2-5 monolayers). The examples of Raman spectra registered shown in the Figure 5, which demonstrates the decrease in difference ( ) for MoS2 flakes deposited from ligand – containing solution by ~1.3 cm-1. Similar results were also obtained at 488 nm excitation. These results indicate exfoliation of MoS2 without and with the ligands penicillamine ligand, with greater exfoliation taking place due to the presence of the penicillamine. These results match well to the previous thicknesses of MoS2 sheets obtained using AFM, confirming that our samples consist of few layer MoS2. It should also be noted that Raman analysis has given thicknesses measurements lower than AFM, though due to the inherent polydispersity in the sample as expected. XPS was carried out on cysteine and penicillamine functionalised MoS2 samples and the data are shown in Figure 6. From this we can identify MoS2 using the Mo 3d XPS spectra ranging from 222 eV to 240 eV. In addition, in this range we also observe the presence of a MoOx, indicating that some oxidation has taken place during the liquid assisted exfoliation, though this has been reported to be produced via conditions of liquid assisted sonication75 and has even been promoted to facilitate exfoliation76-78, therefore it is possible due to the long sonication times used to produce our samples. While the S 2p was studied from the range of 160 to 172 eV, enabling the MoS2 to be identified at between 164-160 eV. Another broad peak was identified in the region from 164 to 170 eV and is assigned to the origin of the sulphur from penicillamine or cysteine ligands present. Therefore, XPS results have enabled us to confirm the attachment of


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ligand to the surface of the MoS2, and in addition allowed us to confirm the chemical nature of the flakes as MoS2, with a small percentage of MoOx detected.

Figure 6. XPS data showing cysteine and penicillamine functionalised MoS2 exfoliated nanomaterial.

Thus, Raman spectroscopy and AFM have clearly confirmed the presence of exfoliated MoS2 flakes, with TEM and STEM microscopy determining the morphology of the nanoflakes when dried. Also FTIR and XPS clearly confirm the presence of ligands on the surface of the functionalized MoS2. Most importantly the use of CD spectroscopy has shown the presence of chiroptical response from the MoS2 due to chiral penicillamine and cysteine ligand functionalization of the nanoflakes. Consequently, our experimental results clearly show that MoS2 nanoplatelets are functionalized with chiral ligands and exhibit different absorption rates of left circularly polarized light and right circularly polarized light.


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Theoretical studies of optical activity of molybdenum disulphide conducted so far have been primarily focused on the valley-selective circular dichroism of infinite monolayers.33, 35, 79 It was previously demonstrated that the optical pumping of such monolayers with circularly polarized light can produce valley polarization, which can be used to control the number of electrons in various kinds of valleytronic devices. However, these studies cannot be used to explain our experimental data, because the circular dichroism of small MoS2 nanoplatelets requires a completely different theoretical treatment. We therefore developed a quite accurate theoretical model of this phenomenon by considering the impact of chiral distortion of the MoS2 crystal lattice on the dissymmetry of optical absorption of the nanoplatelets. Our model relies on the Rosenfeld’s approximation, in which this dissymmetry is described by the rotatory strengths of interband transitions between the size-quantized energy states of the nanoplatelets.


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Figure 7. Schematic presentation of (a) ensemble of randomly oriented chiral nanoplatelets, and the transformation of (b) a chiral nanoplatelet with 40 nm, 32 nm, 4 nm,

and 0.03 nm-1 into (c) an achiral one.

We consider an interband transition from a valence-band state Ψ of a chiral

nanoplatelet to a conduction-band state Ψ , where the subscripts and denote the sets of quantum numbers of the size-quantized states of electrons and holes, and ( ) and

( ) are the Bloch functions at the Brillouin zone center and envelope functions of electrons (holes). The rotatory strength of this transition, averaged over a monodisperse ensemble of randomly oriented nanoplatelets (Figure 7(a)), is given by the Rosenfeld’s formula:80


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Im〈Ψ | |Ψ 〉〈Ψ |" |Ψ 〉#.

(1)

Hereinafter we assume summation over repeated indices for notational brevity. The matrix elements of the electric-dipole and magnetic-dipole moments in a nontrivial metric $

%

are given

by the expressions: 81-82

〈Ψ | |Ψ 〉

〈Ψ |" |Ψ 〉

&' ∗ ) $ "(

√0' 2 2"( 1

%3

%

+% d-,

∗ % 35 ) 4 $ +5 d-,

(2) (3)

where ' and "( are the elementary charge and mass of a free electron, 1 is the speed of light

in a vacuum, 0 is the high-frequency permittivity of MoS2, 2

%3

is the Levi-Civita tensor, is

the transition frequency, and 4 and +% are the components of the coordinate operator and of the

Kane parameter respectively. We next approximate the shapes of all the nanoplatelets by a chirally distorted plain nanoplatelet shown in Figure 7(b). In Cartesian coordinates 4 67, 8, 9: this nanocuboid occupies the region of space

≤7≤ , 2 2

≤8≤ , 2 2

≤ 9 + 78 ≤ , 2 2

where ≫ , ≫ , and || ≪ 2 / # is the small handedness parameter. Upon a

transformation to different coordinates x 7, @ 8, and A 9 + 78, our chirally distorted

plain nanoplatelet turns into an ideal (plane) rectangular nanoplatelet × × of the same


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volume. The envelope wave functions and energy spectra of charge carriers confined by such a nanoplatelet can be calculated analytically using the stationary perturbation theory.83-85 The states of the carriers are determined by sets of three integer quantum numbers, CD , D , D E

and C" , " , " E. Figures 7(b) and 7(c) show the modification of a chiral nanoplatelet upon the considered transformation.

The transformation of coordinates that alters the spatial metrics also changes the behavior of the confined charge carriers inside the nanoplatelets and, as a consequence, modifies their interaction with light. To describe this effect, we calculate the metric tensor in Equations (2) and (3) using its definition through the derivatives of new coordinates F 6G, @, A: with respect to

the old ones 4 67, 8, 9:. By ignoring the small terms proportional to , we get the following

contravariant metric:

$

%

H J

1 IF IF % 0 ≈ L I4J I4J @

0 1 G

@ G N 1

(4)

In calculating the rotatory strengths of interband optical transitions, one should also take into account the symmetry of the crystal lattice of the nanoplatelet by assuming that the Kane parameter +% + , + , + # has generally two unequal components such that |+ | O+ O +P

and |+ | +∥ . With these considerations, the substitution of Equations (2) – (4) into Equation (1) and the evaluation of the matrix elements using the envelopes from the reference

83

lead to the

following expression for the rotatory strength of interband transition Ψ → Ψ in the ensemble of randomly oriented chiral nanoplatelets (see Supplementary):


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√0' C+ +P E ST UVW UVX Y Z 2"( 1 ∥

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(5)

+ T [VX \X ZVW \W + [VW \W ZVX \X Y ZV] \] ^,

where Z_` is the Kronecker's delta, Z ZVW \W ZVX \X ZV] \] , 1 1 21#V UV a + c, 4 3 D b

[V\

8D" 1 Re & Vh\hi . b D " #

The obtained expression shows that the nanoplatelets’ CD spectrum involves both the lines of the dipole-allowed transitions between the states of the same quantum numbers (D " ,

D " , and D " ) and the lines of the dipole-forbidden transitions, occurring with the change in the parity of the quantum number D or D (e.g. D " , D ≠ " , and D " ). These spectral lines are centered at frequencies: ℏ

b D D D b " " "

l + a + + c+ a + + c, 2"m 2"n

where l 1.23 eV is the band gap of MoS2 and "m and "n are the effective masses of electrons and holes. The theoretical CD spectrum is given by the equation CD# ∝ H ,

Γ/b# , # + Γ

where Γ is the half width at half maxima of the spectral lines and the summation is taken over all the relevant electronic transitions. It should be noted that the rotatory strength scales in proportion to the handedness parameter . Therefore, one can take into account the averaging of


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over all the possible twists of the nanoplatelets by replacing with a certain effective

parameter stt .

As Eq. (5) for the rotatory strength suggests, the CD of chiral-shape MoS2 nanoplatelets is due to the mixing of different size quantized states of excitons upon the distortion of the crystal lattice. The optical activity of our nanoplatelets is therefore similar to the optical activity of chiral plasmonic nanoparticles, 86 which comes from the mixing of different plasmonic states due to the chiral distortion of the nanoparticle shape. Figure 8 shows the comparison between the experimental and theoretical CD spectra of MoS2 nanoplatelets exfoliated in the presence of two kinds of chiral ligands. In order to achieve a good fit of the experimental data, we take into account the splitting of the valence band into the lighthole (lh) and heavy-hole (hh) subbands, with effective masses "uv and "vv . The simulation

parameters are: ℏΓ 0.1 eV, "m 0.3"( , "uv 0.4"( , "vv 0.7"( ; 16 nm,

20 nm, and 2.88 nm ( is equal to four monolayers of MoS2). The best agreement

between the theory and experiment was achieved when the ratio of parameter +∥ +P of heavy holes to the same parameter of light holes was 0.6 for penicillamine and 1.7 for cysteine. The difference of this ratio for the two kinds of ligands can be explained by the different interaction strengths between their dipole moments and the dipole moments of the MoS2 unit cells.


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Figure 8. Comparison between [(a) and (b)] theoretical and [(c) and (d)] experimental spectra of MoS2 nanoplatelets exfoliated in the presence of penicillamine (left panels) and cysteine (right panels) molecules.

Thus, despite some assumptions used in our model, Figure 8 shows a very good agreement between the theoretical and experimental spectra. Furthermore, the model allows us to identify the origin of the CD peaks as follows: peaks (i) and (ii) are due to transitions CD , D , 2E →

C" , " , 2E from the subbands of heavy and light holes respectively, whereas peaks (iii) and (iv)

correspond to transitions CD , D , 3E → C" , " , 3E from these subbands. Transitions


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CD , D , 1E → C" , " , 1E do not show up in the CD spectra, as they fall out of the experimental wavelength region. It is also important to mention that the theory we used for the interpretation of the experimental CD signal has been be successfully applied to nanocrystals of more complicated shapes, including chiral nanoscrolls

59-60

and nanocrystals with chirally distorted

crystal lattices 61-63.

Conclusions

In this work, we have developed an approach for the production of chiral MoS2 based 2D nanostructures by the modified exfoliation technique. We expect that due to their high aspect ratio planar morphology, chiral 2D nanomaterials could offer opportunities for the development of chiroptical sensors, materials and devices for the emerging area of valleytronics. Also, potentially 2D nanomaterials are excellent nano-building blocks to develop high-performance separation membranes as these nanomaterials exhibit extraordinary permeation properties, enabling ultra-fast and highly selective membranes for separation and purification.

87-89

For

example, it has been shown that the graphene oxide based membranes can offer precise, superfast sieving of ions and molecules.

90

Therefore, we believe that chiral 2D inorganic

nanomaterials could be highly promising as potential materials for the fabrication of membranes for chiral resolution of technologically important enantiomeric molecules (e.g. drugs, biomolecules, food additives, fragrances, etc.). Thus, further systematic experimental and theoretical research is necessary for a fundamental understanding of the structure, properties and


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behavior of chiral 2D nanomaterials and the development of their technological applications in the near future.

Methods Starting materials and chemicals L- Alanine (99.5%), Cysteamine (98%), L-Cysteine (98.5%), D-Cysteine (99%), L-Cysteine ethyl ester hydrochloride (98%), L-Cystine (99.7%) , L-Glutamic acid (99.5%), L-Glutathione reduced (98%), MoS2(99%, particles size~6 µm, max. 40 µm), L- Penicillamine (99%), DPenicillamine (98-101%), D-Penicillamine disulphide (97%), 2-Propanol (99.9%), MPA, (3mercaptopropionic acid), L-Methionine (99.5%) (99%) and WS2 (99%, particle size 2 µm) were all supplied by Sigma-Aldrich and used without purification. Millipore water was supplied in house and used as supplied.

Instrumentation Sonication liquid assisted exfoliated of MoS2 was carried out using a Grant XUB5 100W sonic bath of 4.5 L.TEM images were obtained using an FEI Titan Transmission Electron Microscope at an operating voltage of 300 kV. Samples for TEM were prepared by dropping a small aliquot of a solution of the QDs in hexane onto a lacey carbon 300 mesh Cu TEM grid followed by drying at 70 °C for 2 hours. SEM was carried out using a Zeiss Ultra plus SEM.UV-Vis absorption spectra were recorded using a Varian Cary 60 UV-Visible Spectrophotometer and a a Perkin Elmer LAMBDA 1050 UV/Vis/NIR Spectrophotometer, while photoluminescence spectra were recorded using a PerkinElmer LS 55 Fluorescence spectrometer. All spectra were corrected using calibration data provided by the manufacturer. Circular dichroism measurements


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were performed using a Jasco J-815 spectrometer. AFM images were acquired in tapping mode using the inverted configuration of the NTEGRA Spectra (NT-MDT). NCH tips (Nanoworld) with a resonant frequency of approximately 275 kHz were employed for capturing the images. Images were analysed using the gwyddion software package. Raman- Raman scattering measurement were carried out at room temperature using Renishaw RM1000 micro-Raman system equipped with Leica microscope (with 50x objective lens), Peltier cooled CCD camera, 1800 l/mm grating, 633 nm excitation wavelength from He-Ne laser and 488 nm excitation from Ar+ ion laser. The laser power was kept low (at ~ 1.5 mW on the sample) in order to avoid the sample heating. Raman spectra were registered in back scattering configuration with vertically polarized incidence light; and no polariser was used in the scattering channel. DRIFT -IR – find instrumentation information. XPS was carried out using a VG Scientific ESCAlab Mk II system with Al Kα X-rays. This was performed in ultra-high vacuum conditions (

Induction of Chirality in Two-Dimensional Nanomaterials: Chiral 2D

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