Recent advances in experimental basic research on graphene and graphene-based nanostructures

(1)

I ttt

Review

Recent advances in experimental basic research on graphene and graphene-based nanostructures

Van Hieu Nguyen'-^

Tixhaology VAST, 18 Hoang Quoc V H Cm Giay, Hanoi, VieBan

,^'£^t^^SZ'^''"'"'^''-^'^'"'''^^"-'''"''''"^''^'''^''^-^y.

E-mail: nvhieu@,op.vast.ac.vD Retaaved 2 Febraary 2016 Accepted for publication 10 March 2016 Published 28 April 2016

Abstract CrossMuk m e p ^ t „ o ± is a review of the lesulls achieved in the experimenal basic research on

Mowing rapidly developing modem topics of nanoseienee and nanotechnology related to gmphene and gmphene-based nanosystems: leduction of graphene oxide and investigaaon of physical propemes of reduced graphene oxide; fabrication and invesdgadon of giaphene q — u m dots; study of light emission from excited graphene; f a b n c a L and i n ^ t i g r d o n of graphene nanopores; prepaMon and invesdgadon of graphene oxide-liquid caystals ts well as Z ^ ^ ^ i T ° dispersions. Besides presendng the «;ie„dfic content of die above- raenooned five topics m detail, we briefly mendon promising and interesting worits S T m ^ C e : , * ' ' " ^ "' """^ " ' ' = " " •" « " ' " ' " = » " 8-Plene-b-ed nanosductuies is R e p o r t s : quanmm field, Dirac ferinion, elecdomagnedc field. Green's fimerion, pertuitadon Classificadon numbers: 4.00, 4.01, 5.15

1. Intitxiuction . ,

corttucopta of new physics attd paiential applications', as was Soon after the discovery of the two^iimensional 2a.s of ' " ? " ' ' ' ° ' = ™ " " f * " ™ ' * ' W - A n evidem demonsdadon massless Dirac fennions^i graphene ^ T o v r i t . ' S e i ^ ' d „ y T h " ' " T " ' ; " ' ™ ' " " * " ' " " ' " ' " " " " ' -

" ai 111 and the subsequent experimental observadon otZ ^ , 1 ^ 1 ™ t ° rf r T " ' f""""' ' " " " " " " " ^ quannm. Hall effect and Beny phase in graphene by Kim et al , ^ ° "' *= ^"^'•'"''-I'Med sensor [5]. The [21 as weU as die demonsnauon of ch£d t n m i e ^ l ^ d die T '"°"'"' °' '^' ^"P"™''"'"' " ^ ^ ' ^ °» "1=

Klein paradox in graphene by Katsnelson, Novoselov and , ^° " "» '=''=«"'^»' conductivity due to adsortxd gas Gem, [31, die research acdvides on graphene have e m e r S D°„ " T * " " " "''""°''-

like 'a rapidly ming star at, Ihe horizon afrmtlerials science k ^ ' """'• "^ " " " ' " ' ' " " »PPlicadons of arid cortdensed-rmner physics', and graphene has .evealed 'a ^ ^ f ™ * ' " mcreased from bemg a domain of condensed- '™""'P''>'"" an''materials scence to also being explored in l » M l M « e » 1 ">••« tam Um woAm,, 1,. „«d ..derihe icm "1'=""™ engineering. In panicular, Schwierz has published a

• l | < i a i a i | of ft. c«ni«, Comma. Aaiib.do, 3.0 ! « „ „ Any ""iproliensive leview on graphene transistors 16]. A review r S , ' r r ^ ' t : ; t ; r : ™ S S , ' ° * - - - " " " • " " • ° * ' - ° " ^ ' • 7 - ' > » - ' optoelectronics, plasmonics L d phoToil™

was recendy presented in [7]. Graphene can also be efficiendy

(2)

Adv. N a t SCI.: Naoosci. Manolechnor. 7 (2016) 023001

applied in photocatalysis. hi [8] Jaroniec et al presented a review on graphene-based semiconductor photocatalysts.

Recent advances in the research on graphene-based photocatalysis, which were achieved after the submission of the review [8], were presented in [9].

Beside the active study on the apphc^ons of graphene, during recent years, die basic research on gr^hene has achieved promising results. The purpose of die present work is to review recent advances in basic experimental research on graphene as well as on graphene-based nanostructures and nanomateriais.

In section 2 we present the results of the experiments on the reduction of graphene. Section 3 is devoted to Uie review on the fabrication, characterizations and luminescence study of graphene quantum dots. The subject of section 4 is die light emission from excited graphene. hi section 5 we present the results of the research on graphene nanopores, and section 6 is devoted to the study of graphene oxide-liquid crystals and aqueous graphene oxide dispersions. The conclusion and discussions are presented in section 7.

2. Reduction of graphene oxide and investigation of some physical properties of reduced graphene oxide

The reduction of graphene oxide (GO) was efficiendy performed by two different mediods: die thermal reduction [10]

and the Birch reduction [11]. In [10] Banerjee ei al con- jectured that on anneaUng, die random epoxy gi«ups in the native GO migrate over die GO surface by acquiring diermal energy and self-assemble to form several long chains of epoxy groups. Subsequentiy, upon diermal reduction die GO sheet is unzipped along diese long chains giving rise to moving zigzag edges, resulting in the enhancement of die magnetization The authors also found diat die density of die epoxy groups plays an important role in the unzipping process. If Ehe density of die epoxy groups is low, dien unzipping of GO is nol possible. The chemical reduction of GO also does not favor unzipping.

In die interesting article [12] Sofer, Pumera el al s

interference device (SQUID) magnetometry. For the fiist time, the authors demonstrated that the Bireb reduction of graphite oxides can lead to highly hydrogenated grapheaiBs.

TTie investigation on the magnetic properties showed that this material has an intrinsically complex stmcWre, consisting of both ferromagnetic and antiferromagnetic components.

Recendy, Tamiguchi, Yokoi el al [16] applied flie photoreduction mediod to reduce GO without usmg additional chemicals and investigated the ultraviolet-visible (UV-vis) absorption, die steady-state and time-resolved PL in the visible-near infrared (NIR) range, and die magnetic properties of reduced graphene oxide (rGO).

The black-colored rGO dispersion obtained after photoreduction for 6 h mamtamed high colloidal stability, while photoreduction for 40 h resulted in the precipitation of hydrophobic rGO sheets. The absorption mtensily in the visible range increased with the mcrease of the photo-ura- diation duration, where die optical enei;gy gap shifted towads die low-energy side from 2.9 to 1.5 eV. Raman spectioscopic analysis showed diat die integrated intensity ratio of die D band at ~ l 3 5 0 c r a " ' to die G band at ~1600cm~' (ID/IQ) slighdy increased after photoreduction for 6 h,

PL measurements were performed to further investigate die effect of photoreduction on GO electronic states. Unre- duced GO induced a broad PL band at ~650 nm due to the TT- TT" transition. As die photoreduction progressed, PL was red- shifted and weakened. The NIR PL properties of GO and photoreduced GO were investigated by steady-state and time- resolved measurements.

The optical investigations suggested diat photoreduction of GO introduced localized levels. If these levels come ftDin local stmctures widi unpaired electrons, dien diey would afford localized spin moments and dius die magnetic properties of GO, The audiors investigated die mfluence of photoreduction on die magnetic properties of GO using a superconducting quantum mterference device. It was found diat GO displayed diaraagnetic behavior at room temperature, while a paramagnetic signal was predominandy observed for the rGO sample.

The local structure of rGO was investigated using C„

tematically evaluated die suitability of GOs prepared by XPS, tiansmission electron microscopy (TEM) " C sohd- vanous standard mediods such as die Staudenmaier [13],

Hoffman [14] and Hummers 1151 medtods to undergo BnclJ reducSon (111 using Na as die electton donor and medianol as die proton donor. Tlie authors invesogated die nature of Bireh-reduced GOs by using various material charactenzation mediods such as scanning electron microscopy (SEM) energy-dispeniive x-ray specdoscopy (EDS), Founer-trans-'

state nuclear magneuc resonance (SSNMR) and FTIR specroscopy in order to seek die origins of the photoreduc- oon-included change in the optical and magnedc properties.

Ab initio calculauons were also carried out to investigate how die formation of C-H bonding and carbon vacancies affect die optical and magnetic properties of an sp^ nanodomain. Integrating all tile calculated and experimental results, it fnTT„ :„f^ J ^ ,w un,,a- "'"-B".iu,g du uic eaicuiatco auo CXD

™ c ™ : V P ^ 1 ™ = , ' ™ . ^ . _ ^ . ? ^ f ^ ° - " » ° " '» ' - • ^ ' « .•" - p i " ' " * = Pholoreducdon' spectroscopy (XPS). combustible elemental analysis, energy-

dispersive x-ray fluorescence spectiDscopy (ED-XRF) inductively-coupled plasma optical emission spectroscopy aCP-OES). Raman spectroscopy, photo luminescence (PL) measurements and elecnical resistivity measurements The magnetic properties of the hydrogenated graphenes (gra- phanes) prepared via die Birch reduction of graphite oxides were invesngaied by using superconducting quantum

-induced modifica- tions of bodi the optical and magnetic properties in terms of die hydrogenation of die sp^ nanodomain surf'ace and vacancies without contradiction.

In [17] Rea et al investigated die enhancement and wavelength modulation of die PL spectrum of GO sheets infiltrated by a spin-coating technique into silanized meso- poraus silicon (PSi). The chemical natiire of GO was confirmed by Raman spectroscopy: the broad G and D peaks and

(3)

7 (2016) toaooi

die kntf-inimis^ 2D and D-Ki bands cbaiactaistic of GO veK cleariy visible.

I h c dieaikal conqx)sidon of die hylxid sttutSure was investi^ted by FIDt s^jecttoscopy. The PL sipial emitted from die GO nanosbetts infiltrated in PSi was invKtigated at tbe excit^cHi waveleaigfli of 442 nm. Rraults were i^xmed lo^hCT wifli PL emission of bare silanized PSi and of GO qrin-coated CMI silanized crystalline silicon, for comparisoiL It was obsCTved dial after infiltration in PSi, die PL signal emitted from GO was enhanced by a fector of almost 2.5.

This sinmg odiancement was mtribuled to die high GO

«incraitrai(m inside die spon^-hlre PSi stracOire. Moreover, die modtdadoD of die pht«olummescence signal was also obsaved. This wavelengdi modulation of GO PL opened a new pCT^)ective for GO exploitation m innovative optoelec- dwiic devices and highly sensitive fluorescence sensors.

3. F^Mcation, characterizations and iumines<%nce staidy of graphene quantajm dots

Gi^ilraie quantum dms (GQDs) were known to have fesci- nMing optical and electronic properties. In several recent cxperimenial worits GQDs having various sizes, shapes and chemical compositions and dierefore displaying a high het- CTogeoeity woe f^mcaled, and tiieir material characterizations as well as dieir iummescence properties were investigated. In [ 18] Ajayan et al demonstrated fliat during die acid treatment and chemical exfoUation of traditional pitch- based cart»n fibers, die stacked graphitic submicTDmeter domauis of die fibers weps easily broken down, leading to die creation of GQDs widi different size distribution m scalable amramts. The as-prodmred C}QDs wifli tiie size range of 1-4 nm showed iwcndimensional morphology, most of which exhibited zigzag edge structure and had a 1-3 atomic layer thickness. The PL of GQDs was tailored dirough varymg dieh sizes by d i a n ^ g die process parameters. Due to die PL siabdily, nanosecond lifetime, and biocompatibUity GQDs were demonstrated lo be excellent probes for high-contrast bioimaging and biosensmg appUcations.

A fadle synflietic mediod for pristine GQDs and graphene oxide quantum dots (GOQDs) was elaborated by Cho etal (19J. The structures were syndiesized by chemical exfohation horn tbe graphitic nanopartides widi high uni- fonnily m terms of shape (chcle), size Oess dian 4 nm) and duckness (monolayer). The physical origin of die blue and green PL of GQDs and GOQDs was attributed to mtrinsic and extrinsic energy states, rrapectively.

Grecnish-yeUow luminescent GQDs witii a quanOim jidd (QY> up to 11.7% were successfidly fabricated via cleaving GO under acidic conditions by Zhu ei al [20] The cleaving and reduction processes were accomplished simul- raneousiy usmg microwave treatment without additional reducing ageoL When die GQDs were fimher reduced widi NaBH4 bright blue luminescent GQDs were obtained widi a QY as high as 22.9%. Bodi GQDs showed well-known exatancHhdcpendait bdiavior, which could be ascribed to die

•ransmon &Dm die lowest unoccupied molecular ortiital

(LUMO) to die highest occupied molecular orbital (HOMO).

Electrochemiluminescence (ECL) was observed ftom die GQDs for die first time, suggestmg promising ^plications in ECL biosensing and unaging. Tlie ECL mechanism was mvestigated in detail. Furtiiramore, a novel sensor for Cd^"^

was proposed based on Cd^+ induced ECL quenchmg widi cystem (Cys) as the masking agent.

A hydrodiermal rouK for cutting graphene sheets into blue luminescent GQDs was demonstrated by Wu el al [21].

The audiors prepared water soluble GQDs wifli a diameter of ca. lOnm diat exhibited blue PL by die hydrodiermal (chemical) cutting of oxidized graphene sheets. The mechanisms of die cutting and luminescence were discussed. This discovery of PL of GQDs might extend tfie range of application of graphene-based nanomaterials to optoelectronic and biological labeling.

In [22] Sun el al discussed the common origm of green PL in cariron nanodots and GQDs. Carbon nanodots (C-dots) syndiesized by electrochemical ablation and small-molecule carbonization togedier widi GQDs fabricated by solvodier- mally cutting graphene oxide are two kmds of typical green fiuorescence carbon nanomaterials. Insight mto die PL origm of diese fluorescent carbon nanomaterials is one of die important topics of nanophotonics. In diis article, a common origin of green luminescence m diese C-dots and GQDs was investigaed by ultiafast spectroscopy. According to die change of surface functional groups during surface chemical reduction experiments, which were also accompanied by obvious emission-^Tpe transform, tiiese common green lununescence emission centers diat emerged ui diese C-dots and GQDs syndiesized by botiom-up and top-down mediods were unambiguously assigned to special edge states consisting of several carbon atoms on die edge of carbon backbone and functional groups widi C = 0 (carbonyl and cartioxyl groups). The obtained findings suggested dial die competition among various emission centers (bright edge states) and t r ^ s donunated die optical properties of tiiese fluorescent cartion nanomaterials.

The physical origin of die green fluorescence of GQDs is an interesting problem of graphene photonics. In [23] Wang ei al studied dus problem by a combined usage of femtosecond transient absorption spectroscopy and femtosecond time-rcsolved fluorescence dynamics measured by a fluorescence upconversion technique as well as a nanosecond time- conelated smgle-photon counting technique. The audiors have found a fluorescence emission-associated darit intrinsic state due to die quantum confinement of in-plane fimctional groups and two charactenstic fluorescence peaks diat appeared m all samples and were attiibuted to independent molecule-like states. Tins finding established flie correlation between die quantum confinement effect and die molecule- Uke emission m die green fluorescent GQDs, and might lead to mnovative technologies of GQD fluorescence enhancement as well as its mdustrial application.

The upconversion fluorescence in C-dots and GQDs was discussed in a recent interesting work of Wen ei al [24] The audiors mentioned diat in many previous worics die upconversion fluorescence was frequently considered as an

(4)

A*>. rteL So. rtenosei. Nanotechncri 7(2016,0230)1

important feamre in C-dots and GQDs, and some mechanisms as well as potential applications were proposed. In conttast to such a general helief, die audiors demonsdnted tiiat no upconversion fluorescence based on five different synthesized C-dos and GQDs was observed. The audiors confirmed tiiat die so-called upconversion fluorescence acdially originates ftom die norma] fluorescence excited by die leaking comp- onent from tile second difiraction in die monochromator of die fiuorescence spectrometer. Upconvenaon fluorescence can be identified by measuring the excitation intensity dependence of the fluorescence.

In [25] Roding et ai petfonned tile fiuorescence lifetime smdy of GQDs. The heights of die GQDs widi die largest value, about 1 nm, were detennined by means of atomic force microscopy (AFM), while dieir sizes witii die diameter dis- nibntion in ttie mteival d = 8.3 ± 2.9 nm were measured by using the TEM images.

Steady-state emission spectia of GQDs in die i3nge 350-700 nm were reconled by a dme-resolved PL speedo- meter. At tile emission mtensity maximum A ^ 430 nm tile fluorescence hfetime was measured by using time-correlated smgle-phomn counting (TCSPC). For each of die GQDs and fluorescent hfetime data sets, die following four models were fitted: die monoexponenda], die sdetched exponential, die log-nonnal distribution and die mverse gamma disdibudon models, and tiie estimated values of die mean and sdmdart deviations were stuthed.

For comparison, a simulation stiidy was also carried out.

The autiiors compared a,e computational speeds and smdied asymptotic bias m estimated paramcleis when die model was misspecilied. It was found diat die difference in die estimated values of die mean and stimdaid deviations for differem models could vary considerably and more so for stiongly non- exponential decay.

A fully dansparent quantimi dot-Ught emitting diode (QD-LED) mtegrated wiUl a graphene anode and cadiode was fabricated by In el ai for tile fiist time [261. The autiion, used die giaphene films widi conttoUed work taction and sheet resisdmce for botii die anode and catiiode. The fabrication process was petfonned as follows: I) fonnation of tiie graphene anode by die diy-dansfer metiiod; 2) fabrication of active layers by die spin^oadng metiiod: 3) fonnation of tile graphene caUiode by Oie dry-oansfer mediod and dry etchinit through which die emissive areas are defined as die over- lapped ones between die cadiode and die anode

Eitber gold nanopaiticies or silver nanowirea were .nsened between layet, of giaphene to condol die work fltncdons, whereas die sheet resistance was detennined by tile number of giaphene layen. The inserted gold nanopartides or stiver nanowires in tile graphene fihns caused a charge ttansfer and changed die worit ftmction to 4 9 or 4 3 eV respectively, Irom Uie original work fiinction of 4 5 eV in tile Uie anode and d,e catiiode were also improved significandv when die number of graphene laye„ inereLed. TtJTl7l of die optictd characteristics such as Raman spectia, ttTs mitmce at tiie wavelengdi of 535 nm coriespondi^ t o " e peak of die elecdoluminescenc. (EL) and tile v l g e

dependence of EL spectra were i:arefiilly investigated by I authors.

Tile alleviation of immune-mediated liver damage iisi large GQDs was proposed by Volaievic et ai [21]. For t first time, these authois demonstrated the immunomoiluiatQi and cytoprotective effects of GQDs in a mouse model i unmune-mediated liver damage: GQDs alleviate in mediated fulminant hepatilis by reducing hepatic inflau oxidative stress, apoptosis and autopbagy. TTie obscvm effects apparendy involved bodi immunomodulatoiy airti«

exerted via tiie intijrference witii T ceU and macrophaf activation as well as direct hepatoprotective action due to HVi accumulation.

4, Light emission from excited gr^ptiene In nxent years, tiie research on die enussion of r a d i a t i o n ^ different wavelengdi ranges was petfonned in s e w ^ experimendd works. The spatially resolved diermal radiad^

emitted fiximelecttically biased graphene was investigated by Freiag el al [28]. These audiors have demonstrated how to exttact die mfonmation on temperatiire distribution, cania densities and spatial location of die Dirac point in die gis- phene channel fltim die experimental data. It was shown tlBt die graphene exhibits a temperBtiire maximum witii a locadott"

tiiat can be conttolled by Uie gate volage. Stationary hot spots were also observed. Thus, die infrared emission can be as«4i as a convenient and non-mvasive tool for die characterization S

of graphene devices. ^ In [29] Berciaud et ai examined ttie inoinsic enei©'

dissipation steps in electtically biased graphene channels, l y , combmmg in sllu measurements of tiie spontimeous opdial emission widi tiie Raman specttoscopy smdy of die graphene sample under conditions of cuiiem flow, die autiion obtained mdependent infonnation on die energy disttibution of Ito, elections and phonons. The elections and holes conttibuting' ti> Uie light emission are found to obey a diennal disttibution, wiUi lemperanires m excess of 1500 K in die regime of cu^

rent sannstion. The zone-center optical phonons are also highly excited and are found tt) be in equilibrium widi die electrons. For a given optical phonon tijmperamre, tto anhannomc downshift of Uie Raman G mode is smaller diin expected under equilibrium conditions, suggesting diat die.

elecffons and high-energy optical phonons are not fiilly equdibrated witii all of die phonon modes.

Altiiough graphene has no band gap and dierefore PL Is not expected from relaxed charge carriers, graphene exdteii by ultiashort laser pulses can emit PL light. In [30] Lui a ol have observed significant light emission ftom graphene under exciation by 30 fs ulttashort laser pulses. Light emission was found to occur across tile visible specttal range of 1.7-3.5 eV witii emitted photon energies exceeding diat of die excitation laser (1.5 eV). The emission exhibited a nonfinear dependence on die laser mtensily. In two-puhte coneladon measurements a dominant relaxation time of lens of femtoseconds was observed. ITie experimental data can be explained by a

(5)

AftcrtetSd.:rtenoBclNanotedmct 7 (2G16) 02300,

twMcmpeiaoire model describing die elections and dicir (^ interaction widi sttongiy coupled optical phonons.

j^ in [31] Pop er al perfoimed imaging, simulation and

^ ekcdostaricconrrol of power dissipation in graphene devices.

^ Ttte andiois ditecdy image hot spot ftmnadon in fimctioning

^ mono- and bikyer graphene field effect ttansisttins (GFETs)

^ asmg inftared dieimal micioscopy. Conelating widi an elec-

^ ttical-diennal dansport model provided insight intia carrier

^ distrihnion. fields and GPET power dissipation. H e hot spot cofiesponded to die location of mimmum charge density

^ along die GFET. By changtiig die appfied bias, Otis could be shifled between clecttodes or held in die middle of die channel in ambipobr ttansport. The audiors noted dutt die hot spot shape bore die imprint of Oie density of stiUes in hUayer graphene. They ako found diat Oiennal imaging combined widi self^»nsistent simuUtion provided a non-invasive appioach for more deeply examining ttansport and energy dissipation in nanoscale devices.

In a recent interesting worit [32] Bae el al observed die bright visible light enussion ftom electticaUy biased suspended graphene devices. It was known diat in diese devices die heal mmsport was gready reduced [33]. Therefore, hot electtons (-2800 K) became spatially locaUzed at die cenKr of die graphene layer, resulting in a 1000-fold enhancement of die radiation efticicncy compared to dial ot die Uiennal radiation [1, 2J.

Freely suspended graphene is largely mrniune to undc- stable vertical heat dissipation [33] and exttmsic scattering effects [34], and dierefore promises much more efficient and brighter radiation in Oie infiared-ttKvisible region. Due tt) die Stiong umklapp phonon-phonon scattering [35] die diennal conductivity ot gnqihene at high temperanne 1800 ± 300K is gready reduced ( ~ 6 5 W m - ' K^'), which also suppresses lataal heat dissipation, so hot elections (~2800K) become spatially localized at die center of die suspended graphene undc^modest electtic field ( - 0 . 4 ^ - ' ) , gready incnasmg die efficiency and brighmess of die light emission. The bright mible dtennaUy ennded Ught interacts witii die reflected light from die separate substtate surface, giving interfenmce effects diat can be used to nme die wavelengdi of die emitted fight.

The auOiors observed bright and sttdile visible tight emission ftom hundreds of electtically biased suspended graphene devices. The emitted visible tight was so intense dial ,t was visible even to die naked eye, widiout additional magmficanon. An anay of electtically biased multiple pand- lel-suspended chemical vapor deposition (CVD) few-layer graphene devices exhibiu»i multiple bright visible tight mnssion under ambient conditions. The observation of stable bright visible light emission ftom large-scale suspended CVD graphene airays demonsdated die great pottntial for die na- , 7 ^ ™ , ° ' * ° complementary melal^ixide-semiconductor (CMOS><ompatible, laige-scale graphene Ught emitters in dijilay modules and hybrid sUicon photonic plati-onns widi mdustty vacuum encapsulation technology.

A microscopic view on die ulttafast PL ftom photo- excited giaphene was presenttd m a recent wori! of Matic

« rd [36]. The audlon, perfonned a joint dieoiy-experiment smdy oo dns ujpic and revealed tivo distinct mechanisms

behind die occmring PL. Besides die well-known nicoherent contribution driven by nonequiUbrium cairiei occupation, die anOiois also found a coherent part tiiat spectially shifted ividi die excitation energy. For die first time, die anOlon, demon- stiattMi die predictiMl appearance and spectial shifts of die coherence PL.

5. Graphene nanopores

For locafizuig and detecting single DNA or protem molecules It was expected tiiat die sofid-stiite nanopore devices might be efficient took [37]. In [38] Krfl a al proposed Oie design of fimcdonatizcd nanopores in graphene monolayen, and showed by molecular dynamics sunulations diat tiley provide highly selective passage of hydnttcd ions. Only ions diat can be partly sttipped of Oieir hydration shells can pass ttirough diese ulttasmall pores witii diametiis of ~5 A. For example, a fluiMine-nittogen-tenninated pore allows die passage of L i \ Na+ and K+ cations widi die ratio 9:14:33, but it blocks tile passage of anions. The hydrogen-tennmated pore afiows Oie passage of F " , Cl and Br anions wifli die ratio 0:17:33, but it blocks die passage of cations. The autiiors predicttjd tiiat diese nanopores could have pottntial appUcations til molecular separation, desaUnation and energy storage systems.

The use of graphene widi nanopores as a subnanometin- ttans- electtode membrane was discussed by Garaj el al [39] Sub- sequentiy, Bashir a al [40] proposed to use nanopore sensors for nucleic acid analysis.

For creating exttemely smaU pores til graphene witii attimic pnKision Golovchenko a al [41] developed an efficient metiiod: die atom-by-atom nucleation and growth of graphene nanopores. It consists of creating defect nucleation centers by using energetic ions, followed by edge-selective election recoU sputtering. As a result, die autiiois successfiiUy creattxi graphene nanopores wiUi radU around 3 A conesp- onding to 10 atoms removed. The auOlors observed carbon atom removal from tile nanopore edge in sin, ushig an aber- ration-conected election microscope. This approach did not njqmre focused beam and aUowed scalable production of smgle nanopores as weU as anays of monodispeise nanopores for atomic-scale selectively penneable membranes.

The shaiply peaked pore-size disttibution indicated dial tile audiois successfiiUy developed an efficient mediod for generating monodisperse nanopona m seraipemieable graphene membranes mned to select molecules wiOl a spectiic size and stiucture.

T i e graphene sheeB witii nanopores can be used as die ion exchange membranes m desalination ttKhnology By applying die molecular dynamics simulations in [42] Xue e, al mvestigated die selective ion ttansport behavior of electtic-fieldKlriven KCI solution tiu^ough charge-modified graphene nanopores. They demonstiated tiiat die presence of negative charges at die edge of tile graphene nanopore can remarkably unpede tile passage of C r whtie it can enhance die nansport of K*. litis is an indication of die ion selectivity of die graphene nanopores. The auUlors investigated die dependence of tins selectivity on Uie pore size and die loud

(6)

tf NaL SC Nanosci Nanolechno, 7 (20,6) 020(01

Chaise number assigned al die nanopore edge. By adjusting die nanopore diameter and die elecaic charge on die nanopore, a nearly complete rejection of C r can be reaUzed. The electncal resistance of nanoporous graphene, which is a key parametiir to evaluatt: die perfonnance of ion exchange membranes, is found two orders of magnitiide lower Oian cottunercially used membranes. Thus, graphene nanopores are prontising candidates to be used m electtodialysis technology for water desaluiation witii a high pennselectivity.

The experimentid research on selective ionic transport Oirough contioUed, high-density, subnanometi;r diameter pores ui Eoacroscopic suigle-layer graphene membranes was perfonned by Kamik et at [43], Isolated reactive defects were first inttoduced mto die graphene lattice dirough ion bom- bardment and subsequenOy enlarged by oxidative etehuig nito penneable pores witii diameters of 0.40 ± 0 2 4 nm and densities exceeduig 10'^ cm"^ while retitining sttncnira] integrity of die graphene. Transport measurements across lon-iira- diated graphene membranes subjected to in sim etching revealed Oiat tiie created pores were cation-selective at short oxidation times, consistent wiOi tiie electiosmtic repulsion from negatively charged fimctional groups terimnaOng flie pore edges. At longer oxidation times, tile pores aUowed tile ttansport of salt, but prevented die ttansport of large organic molecules, indicative of steric size exclusion.

The heterogeneous sub-continuum ionic ttansport ui sKtisticaUy isolated graphene nanopores was also mvestigatt)d m a recent work of Kamik el al [+4]. The audion demonstiated diat isolated sub-2 nm pores ui graphene exhibited, til contiast to larger pores, diveise ttansport behavioni consisB,M wiOi ion ttansport over a free-energy banier arismg ftom ion dehydration and clecBostatic interactions. Cunent-volBge measurements revealed tiiat tile conductance of graphene nanopores spanned Onee order of magnitiide and diat tiley displayed distinct Unear, voltiige-acdvaK;d or rectified cur- rent-volmge characteristics and different cation-selcctivitv profiles.

The obttnned results demonsttated Oiat sub-2 nm graphene nanopores exhibited diveise ttansport bchavion, Oiat can be explauied by electtostatic and hydration interactions of ions witii die pores and dial are remniiscent of biological chamiel. The pores are dynamic and can change fliett Bans- port characteristics at different timescales. The above-presented results suggested tile pott,ntial of snbK:o„tinnnm nanopores m graphene to act as a new class of synOietic ion chamiels and provided a platfom, for probing sub-continuum ttansport for d)e engineering of die desued selectivity and ttansport characteristic at Oie single-pore level

However, Oiere was less nndeistanding as to whetiier nanoporous graphene is sttong enough to mainttun its inherent ,o tiie reverse osmosis desalination process The mechamcal sttengU, of nanoporous graphene as a desaU^aSn showeTti^ara" " ' " ' ° ' ° " " " " " ' " ' « ! ^^ - ' h - "

.1, mtegnty m revenie osmos,s but 0,e choice ot substtate for graphene ,s cntical to tiiis perforaiance. Using m o l a u t o dynamics simulations and continnum ftactore tnL^^^t:

authors showed that an appropriate substrate wnn opeim smaller than I pm would allow nanoporous grapbeae, widistand pressures exceeding 57 MPa or ten times more "

typical pressures for seawater reverse osmosis, and porosity may help the membrane witiistand even pressure.

A graphene nanopore with a self-integrated oj antenna was fabricated by L ^ ei al [46]. The auln demonstiated diat a nanometer-sized heated spot created photon-to-heat conversion of a gold nanorod resting graphene membrane forms a nanoscale pore widi self-i grated optical antenna ui a smgle step. The distinct plasmt traits of metal nanopartides, which have a unique capal to concenfrate light into nanoscale regions, yield die nificant advantage of parallel nanopore fabrication compiiq™

to the conventional sequential process using an elected beam. Tunability of bodi die nanopore dimensions and ftg optica] characteristics of plasmonic nanoantenna were fiir^

achieved. Finally, die key optical ftmction of die p r e p ^ self-integrated optical antenna on die vicinity of the g r a p h ^ nanopore was manifested by multifold fluorescent s i g ^ enhancement during DNA franslocation, ^

The molecular valves for contitiUing gas phase transperi were fabncated from discrete angstrdm-sized pores in gffi- phene by Bunch el al [47]. TTiese audiore showed diat gas Qm dirough discrete angstrom-sized pores ui monolayer grapbeBl can be detected and dien controlled using nanometer-sized gold clusters, which are formed on die surface of g i ^ h e % and can migrate as well as partially block a pore, hi sampMs^

widiout gold clusters die audiors observed stochasfijji switehing of die raagniuide of die gas permeance, which w ^ attributed to molecular rearrangement of die pore. The fob- ncated molecular valves could be used, for example, to develop unique approaches to molecular syndiesis based M die controllable switching of a molecular gas flux, reminiscent of ion channels in biological ceU membranes and sohd-stalB nanopores.

6. Graphene oxide-liquid crystals and aqueous graphene oxide dispersions

As an attempt to find a superior display for die appUcation to die electro-optical switching, Song el al [48] investigated the optical sensitivity to extemal electtic field of graphene oidde (GO) hquid crystals (LCs) widi controllable alignment ITie sensitive response of die nematic GO phase to extemal stimuH makes diis phase attractive for die above-mentioned puipose.

Onsager's dieory predicted die transition from an isoQopic Ki a nematic phase passing dirough a biphase as die concentra- tion of plate-like GO in colloidal dispersions increases. The location of diis transition depends sensitively on die aspect ratio (AR) diameter/duckness of die plates. Since die AR of GO-LCs with monoatomic thickness can be up to the order of 10000 [49], die biphase was predicted to appear at the con- centration around 0.01-0.1 vol%. In an experiment die authors used the GO flakes which were mostly single-layered and had die average AR of about 3200, The audiois predicted '^

i

(7)

Adv. Iter Scj.: Nanosc, Nanotedwo,. 7 (20,6) 02300,

die concenttations for phase ttansitions from die isottopic phase to die biphase tobe0.04vol%,and from die biphase to die nemadc phase to be O.I7vol%. On die oOier hand, by observing die macroscopic birefringence pattem Oie auOlora dett)nnined die experimentid values of die above-mentioned concenttations of approximately 0.08 vol% and 0.2 vol%.

For detemiining Oie Ken" coefficient, Oie auOiors used a cuvette widi parallel eiecfrodes on two opposite walk and a beam-pad! lengdi of 5 mm. The maximum Ken- coefficient was found to be approxinmttly 1.8 x l O - ' m V " ' , a value exttemely large compared to die values of Oie ordera of 10"^

and 10 ' m V ^ for nitiobenzene and aqueous two-dimen- sional gibbsite platelet suspension [50] as well as of 10""- 10 "mV-^ for blue phase LCs [51-53]. To demonsttatt, die significance of die obtitined large Ken coefficient die audiors prepared an electto-optic device usmg two smiple wire clecttodes segtmsed by 5 mm. This model optical device worked very weU under an appfied volKge of 20 V, alOiough its perfonnance was not comparable to tiiose of commercial LC displays. Hie development of a real GO device requires an intensive smdy to synUiesize a high-concenttation isottopic GO dispersion for highly samrated bnefringence, to conttol die ionic mfluence and precipitation for long-ttim subility, and to develop a new drivmg scheme suitilblc for electiolytt materials.

til die short note [54] die auUiois remarked diat while Uie conventional LC displays tidce die advanage of die orienB- don ftom surface-induced to electtic-field-induced alignmem die electtlc-field switching of GO-LCs occurs Ulrough a direct ttansioon ftom an isottopic to a highly aUgned Uquid-ciys- taUme phase. A high Ken- coefficient stems from die syner- gistic effect ot Ore large GO polanzabihty anisotiopy and die Onsager excluded-volume effect for LC ordering (LC alignment increases ttanslational enttopy at tile expense of rota- tional entropy).

Because of die high shape amsotiopy of GO and tile electiical double layer forined at i c surface, die GO polanzabihty parallel to die plane of die flake is gready enhanced when die extemal electtic field is switched on. The collective aUgnment of GO fiakes occuning at low concenttations also conttibules to Uie large Ken coeflicient. AnoUier advanage of CO-LC displays is tiiett low power consumption However for tile development of GO-LC displays fliere exist several' chaUenges which should be overcome.

In Uie subsequent work [55] Song « al investigated die raatenal propenies and electto-optic response of aqueous GO dlsperaons wiUi varying ion types and ion concenttations rhe material propenies included Uie zetii potential pH and conductivity. The autiiors observed a clear conttast between the NaOH-GO dispersion and GO dispersions wid, otiier added ions. Odier ions drastically desensitized tile electto- optic response of GO dispeniions, but Oie addition of NaOH shghOy enhanced Oie electiical sensitivity of GO dispeisions The audiois mvestigated die underlying mechanisms of flic obtamed resute and clanfied flie ionic effect on bodi flie characteristic conttast between tile dispersed particles and solvem and die surface conductivity of IX). The autiiors demonsttated diat solvent conductivity is unponant for die

electtical sensitivity of (30 dispersions, which mfluenccs Uie characreristic conttast between tiie dispenjed particles and solvent In addition, die audiors uivestigated Uie surfiice electtical characteristic of GO dependtiig on die ions of Uie solvent

The audiois experimentiOly and tiieorctically elucidated die underlymg mechanism of die phenomena. The mechanism is closely relati^ to die acidic nattnc of GO dispersion, which is neuttalized by tiie addition of NaOH. The electii-optic response of GO dispersion was influenced more by Uie electiical properties of tile solvent ratiier tiian by tiiose of die GO particle itself. These results wiU help us to underatimd die electtochenucal and Uquid-ciystaUuie characteristics of GO dispersions and to develop new electto-optic devices usmg these materials.

7. Conclusion anti discussions

In tills article, we have presented a review of recent interesting and promismg basic experimentid works on graphene as weU as on graphene-based nanomaterials and nanos- ttucmres. These works were classified into die foUowuig five

• Reduction of graphene oxide and physical properties of reduced graphene oxide.

• Fabrication and mvestigation of graphene quantimi dots.

• Light emission from excited graphene.

• Fabrication and investigation of graphene nanopores.

• Graphene oxide-liquid ciysttds and aqueous graphene oxide dispersions.

Basic research on graphene has a wide diversity. Besides die five above-presentt»J topics witii impressive scientific content, diere exist also odier promising ones such as electton-phonon coupUngs ui graphene [56-67], graphene nanonbbons [68-78], photodiemioelecttic effect [79-87]

photo-induced doping effect on graphene hettrosttucnires' [88-90], snpereonducting phase in graphene-based hybrids [91-94], cobalt intercalation at tiie interface between graphene and imdium 195], surface-enhanced Raman signals for smgle-molecule magnets grafted on graphene [96], local defonnations and mcommcnsurabiUty of high-quaUty epitaxial graphene on a weakly interacting ttan.sition metiil [97]

confined suites in rotated bilayers of graphene [98], epiaxiai graphene/fcrromagnet hybrids [99], sttains induced by point defects m graphene on mend [100], modulating charge density and melastic optical response in graphene [101] Hj plasma-graphene interaction [102], suppression of grapiiene multilayer patches [103], convergent fabrication of nanoporous two-<Umensional carbon network from an aldol con- densation on meud surface [104], tiiemiodynamic and kmelie aspecB of epitaxial growtii ot graphene [105] ete. Thus die basic research on graphene is stiU conunu,ng to enlarge the scientific content.

(8)

Adv. NaL S o • Nanosd. NanotechnoC 7 (2016> 023001 Acknowledgments

The audior would Uke to express his deep gratitude to die Advanced Center of Physics and histimte of Materials Sci- ence, Vietnam Academy of Science and Technology, for die support. I thank ftof. Le Si Dang (Instimte N&l, Grenoble, France) for his valued cooperation.

References

[IJ Novoselov K S, Geim A K. Morozov S V, Jiang D, Katsnelson M I, Grigorieva I V. Dubonos S V and Firsov A A 2005 Nature 438 197

[2] Zhang Y, Tan Y-W. Stonner H L and Kim P 2005 Nature 438 201

[3] Katsnelson M I, Novoselov K S and Geim A K 2006 Nal Phys. 2 520

[4] Geim A K and Novoselov K S 2007 Nal. Mater. 6 183 [5] Schedin F, Geim A K, Morozov S V, Hill E W. Blake P Katsnelson M I and Novoselov K S 2007 Nal. Maier 6 652 [6] Schwierz F 2010 Nal. NanotecK S 487

[7] Nguyen Bich H and Nguyen Van H 2016 Adv. Nal. Sci • Nanosci. NanaiechnoL 7 013002

18] Xiang Q. Yu J and Jaromec M 2012 Chem. Soc. Rev 41 782 [9] Nguyen Bich H, Nguyen Van H and Vu Dinh L 2015 Adv

Nat. SCI.: Nanosci. Nanotecknal. 6 033001 [10] Bagani K, Ray M K, Satpati B, Ray N R, Sardar M and

Banerjee S 2014 J. Pliys. Cfiem. C 118 13254 [11] Birch A J 1944 J. Chem. Soc. 430-«

112] Eng A Y S . Poh H L, Sanek F, Maiysko M. Matejkova S Sofer Z and Pumera M 2013 ACS Nana 7 5930 [13] Staudenmaier L 1898 Ber. DiscfL Cfiem. Ges. 31 1481 [141 Hoftnann U and Konig E 1937 Allg. Chem. 234 311

""(M339 ^ ^ ^'^ ° ^ ^ " " " ** ^ ^^^^ •'• ^"^ ^''^"^ ^'^•

1161 Taniguchi T el ai 2014 / P/,ys. Chem. C 118 28258 1171 Rea I Sansone L. Terracciano M. De Siefano L, Dardano P

Giordano M, Bomello A and Casalino M 2014 J Phys ' Chem. C 118 27301

118] Peng J « fl/ 2012 Nana Lea. 12 844

^'^^ ^^I- IT^ ^'^' " ^ "• ^'^ '-«• Cho Y-H and Seo T 2013 Adv. Maier. 25 3657 ' " * ' ' ' a . V j i n n ' ? ; ^ r ' ' ^ ' ^ " * ^ - ^ ^ S J R . J i a n g L P a n d 23iu J J 2012 Adv. Fund. Maier. 22 2971 m \^ ' ^ • . ^ ^ ? ^- ^' 2 ^ d Wu M 2010 Adv Mater 11 734 [22] Wang L el ai 2014 ACS Nana 8 2541

[23] Wang L, Zhu S J, Wang H Y, Wang Y F Hao Y W ndi J h ^ g J " ^ d C h e n Q D 2 0 I 3 A r f v 0;,;. Mater. 1 264

'""' ' S c ' n S s ' " " ^ ' ' ' " ' " "'^ Nann T2014.. PHy.

' ' ^ ' 2 ^ 1 4 ' A S ^ ™ ^ . 4 ' ^ ' ^ " - " - ^ ^ - V - ^ H a n d . S [27] Volarevic V el al 2014 ACS Nana 8 12098

^'^^'^Sl^^t?9T"•^-^'"-^----

[29] Benriaud S, Han M Y. Mak K F. Bms L E Kim P and 1301 L^iclL^^'^^"'' ' ' - • ^«-104 227401 " ^ ' [31] B j . M H, Ong Z Y. Estrada D and Pop E 2010 Nano Le..

[32] Kim Y D « al 2015 Nal. Nanoiech. 10 676

[33] Dcigan V E, Behnam A. Conley H J. Boloim K 1 and 2013 Nano Lett. 13 4581

[34] Kim Y D, Bae M H, Seo J T, Kim Y S. Kini H, Lee J H Ahn J R, Lee S W. Chun S H and Park V D 2013 ACSffc 7 5850

[35] Pop E, Varshney V and Roy A K 2012 MRS Bull 37 [36] Winzer T, CiesielsM R, Handloscr M. Comin A,

Hartschush A and Mahc E 2015 Nam Lett. 15 1141 [37] Branton D et al 2008 Nat. BiolechnoL 26 1146 [38] Suit K, Wang B and Kr^ P 2008 J. Am. Chem. Soc 130

16448

[39] Garaj S, Hubbard W, Reina A, Kong J, Branton D and Golovchenko J A 2010 Nature 467 190

[40] Venkatesan B M and Bashir R R 2011 Nat. Nanotech. 6 6 ^ [41] Russo C J and Golovchenko J A 2012 Proc. Nail Acad. M

USA 109 5933 ^' [42] Zhao S, Xue J and Kang W 2013 /. Chem. Phys. 139 1147^

[43] O'Hem S C, Boutiler M S H, Idrobo J C, Song Y, Laoui % Atieh M and Kamik R 2014 Nano Lett 14 1234 [44] Jain T, Rasera B C, Guerrero R J S, Boudlier M S H, '

O'Hem S C, Idrobo J C and Kamik R 2015 Nat. Namteii' [45] Cohen-Tanugi D and Grossman J C 2014 Nano Lett 14 6171 [46] Nam S W. Choi I, Fu C C, Kim K, Hong S, Choi Y,

Zetd A and Lee L P 2014 Nano Len. 14 5584 [47] Wang L. Drahushuk L W, Candey L, Koenig S P, Liu X,

PeUegnno J, Strano M S and Bnnch J S 2015 Nat Nanotedi.

[48] Shen T Z, Hong S H and Song J K 2014 Nature Mater 13 394

[49] Dan B. Behabtu N, Marlmez A, Evans J S, Kosynkin D V Tour J M, Pasquah M and Smalyukh I I 2011 Sofi Maier^J [50] Jimenez M L, Foraasari L, Manlegazza F, Mourad M C and

Belhm T 2012 Langmuir 28 251

[51 ] Hisakado Y, Kikuchi H. Nagamura T and Kajiyama T 2005 Adv. Mater. 17 96

[52] Haseba Y, Kikuchi H, Nagamura T and Kajiyama T 2005 Adv. Mater. 17 2311

[53] Chen Y, Xu D, Wu S T, Yamamoto S I and Haseba Y 201J Appl. Phys. Ult 102 141116

[54] Kim J Y and Kim S O 2014 A^af. Mater 13 325 [55] Hong S H Shen T Z and Song J K 2014 J. Phys. Chem. C

US 26304

[561 Moroso, S V, Novoselov K S, Katsnelson M I, Sctedin F, loro^esm""* "^ " ' ' ° ™ * " ™8 fity-«« iar.

fS! JfT"°' S " ' ' " " "pp™ '' ™* ™)"- K"- l-tK-100 076801 1581 Bo otin K I, Sikes K J, Hone J, Sloraier H L and Km P 2008

Phys. Rev. Lett. 101 096802

159] DasUva A M, Zou K, Jaai J K and Zhu J 2010 Fhys. Rev Let,.

Iu4 236601

[60] Efetov D K and Kim P 2010 Phys. Rev. Lett 105 256805 [61] astro E V, Ochoa H, Katsnelson M I, Gorbachev R V

Ehas D C, Novoselov K S, Geim A K and Gumea F 2010 Phys. Rev. Lett. 105 266601

[62] Huang E H and Das S S 2008 Phys. Rev B 77 195412 6JJ Manani E and von Oppen F 2010 Phys. Rev. B 82 195403 [64] Park C H, Bonini N, Sohier T. Samsonidze G Kozinsky B

Calandra M, Mauri F and Marzari N 2014 Nana Ull.

[65] Chen I J, Mante P A. Chang C K. Yang S C Chen H Y Huang Y R, Chen L C, Chen K H. Gusev V and Sun C K 2mA Nano Lett. 14 1317

[66] Na S R, Suk J W, Ruoff R S, Huang R and Liechri K M 7014 ACS Nano 9 11234

[67] Zhu X. Wang W, Yan W, Larsen M B, Boggild P Pedersen T G, Xiao S, Zi J and Mortensen N A 201-1 A//.™

i^n. 14 2907 "''* nano ,

(9)

M/. NO. Su.: NanoscL Wanotechnol 7 (goie) taaool

[681 Son Y W. Cohen M L and Louie S G 2006 Nature 444 347

^ J BaroiK V, Hod O and Scuseria G E 2006 Nano Lett 6 2748 [70] Yang L. Parte C H, Son Y W.Cohen ML and Louie SG 2007

Phys. Rev. Lett. 99 186801 [71] Cm J etal 2010Nature 466 470

[72] Girao E C, Liang L. Cruz-SUva E, Filho A G S and Mcunier V 2011 Phys. Rev. Lett 107 135501 [73] Rufficux P et al 2012 ACS Nano 6 6930 [74] Bcnneth P B, Pedramrazi Z, Madani A, Chen Y C,

Fischer F R, Crommie M F and Bokor J 2013 Appl Phys iett 103 253114 ^ ' ^ ^ [75] Bronner C, Stremlau S, Gille M, Brausse F, Haase A,

Hecbt S and Tegeder F 2013 Angew. Chem. Int Ed. 52 4422 [76] Chen Y C, de Oteyza D G. Pedramrazi Z, Chen C,

Fischer F R and Crommie M F 2013 ACS Nano 7 6123 [77] Homik Yetal 2014 NaL Maier. 13 57 [78] C^Selal 2014 Nat. Nanotech. 9 896

[79] Wei P, Bao W, Pu Y, Lan C N and Shi J 2009 Phys. Rev. Lett.

102 166808

[80] Lui C H, Mak K F, Shan J and Heinz T F 2010 Phys Rev Lett 105 127404 ^ ' [81] Xu X, Gabor N M, Alden J S, van der Zande A M and

Mc Euen P L 2010 Nano Ult. 10 562

[82] Gabor N M, Song J C W, Ma Q, Nair N L, Taychatanapat T Watanabe K, Taneguchi T, Levitov L S and JariUo-Henero P 2010 Science 234 648

[83] Song J C W. Rudner M S, Marcus C M and Levitor L S 2011 Nano Lett. 114688

[84] Graham M W. Shi S F, Ralph D C. Paik J and Mc Euen P L 2013 Nat. Phys. 9 103

[85] Cai X et al 2014 Nat Nanotech. 9 814 [86] Echtermeyer J I et al 2014 Nano Uti 14 3733 [87] Tielrooij K]elal 2015 Nat Nanotech. 10 437 [88] Kim Y D, Bae M H, Seo J T, Kim Y S, Kim H Lee J H

A ^ JR. See S W. Chun S H and Park Y D 2013 AC5 A'ano

[89] Tibet} A, Rubio-Roy M. PaiUeE M. Huntzuiger J R, Landois M, Mikolasek M, Con&eras S, Sauvajol J L Dujaidin E and Zah^ A A 2013 5cj. ffep. 3 2355 [90] Ju L et al 2014 Nat. Nanotech. 9 348

[91] Alain A, Han Z and Bouchiat V 2012 Nat Mater 11 590 [92] Kessler B M, Girit C O, ZeKl A and Bouchiat V 2010 Phys

Rev. Lett 104 047001

[93] Tonnoir C, Kimouch A, COEMX J, Magaud L, Delsol B Gilles B and Chapelier C 2013 Phys. Rev. Utt. I l l [94] Han Z, AUaln A, Arjmandi-Tash H, Tikhonov K.

Eigel'man M, Sac^p^ B and Bouchiat V 2014 Nal. Phys.

[95] Vlaic S, Kimouche A, Coraux J, Santos B, Locatelli A and RougemaiUe N 2014 App Phys Lett. 104 101602 [96] Lopes Metal 2010 ACS Nano 4 7531

[97] Blan N, Coraux J, Vo Van C, N'Diaye A T, Geaymond O and Renaud G 2012 Phys. Rev. B 86 235439 [98] Trambly de Laissardiere G, Mayou D and Magaud L 2012

Phys. ;;ev. B 86 125413

[99] Coraux J, N'Diaye A T. Rougemaille N, Vo Van C, Kimouche A, Yang H X, Chshiev M. Bendiab N ' Frachart O and Schmid A K 2012 / Phys. Chem. Lea. 3 [100] Blanc N, Jean F. Krasheninikov A V, Renaud G and Coraux J

2013 Phys. Rev. Lett. I l l 085501 [101] Kimouche A, Renault O, Samaddar S, Winkelmann C nn^n ^^""^O'S H, Fmchart O and Coraux J 2014 Carbon 68 73 [102] Despiau-Pujo E, Davydova A, Cunge G, Dclfour L

Magaud L and Graves D B 2013 J. App Phys 113 [103] Han Z er al 2014 Adv. Fund. Mater 24 964 [104] Landers J, Ch^oux F. De Santis M, Bendiab N, Lamare S

Magaud L and Coraux J 2014 2D Maier. 1 034005 [105] Tetlow H, Posthuma de B, Ford I J, Vvedensky D D

Coraux J and Kanlorovich L 2014 Phys. Report 542 195