extrapolates to zero splitting at B =0(21),in contradiction to the data (SOM S7).Integer values of S ≥2can be ruled out on the basis of the temperature scaling described above and becau previous studies of sixfold coordinated Co complexes have found almost exclusively S ≤3/2,with exceptions only for fluoride ligands (1,2).We conclude that only S =1can explain the measurements,and from this we identify the charge state of the metal center to be Co 1+(SOM S8).
In coordination chemistry,the existence of zero-field splittings induced by molecular distortion is well established,but the ability we demonstrate to continuously distort an individual molecule while simultaneously measuring its zero-field split-ting opens the possibility for dramatically more detailed and preci comparisons with theory.For correlated-electron physics,our results dem-onstrate that single-molecule electrical devices can provide well-controlled model systems for studying S ≥1underscreened Kondo effects not previously realizable in experiment.Our work further demonstrates that mechanical control can be a realistic strategy for manipulating molecular spin states to supplement or replace the u of magnetic fields in propod applications such as quantum manipulation or information storage (27,28).
References and Notes
失败是成功之母英语1.A.Abragam,B.Bleaney,Electron Paramagnetic
Resonance of Transition Ions (Dover,New York,1986).2.R.Boca,Coord.Chem.Rev.248,757(2004).
3.N.Agraït,A.L.Yeyati,J.M.van Ruitenbeek,Phys.Rep.377,81(2003).
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8.J.Park et al .,Nature 417,722(2002).
9.M.Grobis,I.G.Rau,R.M.Potok,D.Goldhaber-Gordon,in Handbook of Magnetism and Advanced Magnetic Materials ,H.Kronmüller,S.S.P.Parkin,Eds.(Wiley,Hoboken,NJ,2007).
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Phys.Rev.Lett.97,206601(2006).
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Nature 417,725(2002).
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(2006).
17.M.-H.Jo et al .,Nano Lett.6,2014(2006).18.H.Park,A.K.L.Lim,A.P.Alivisatos,J.Park,
P.L.McEuen,Appl.Phys.Lett.75,301(1999).19.Materials and methods are available as supporting
material on Science Online.
20.J.J.Parks et al .,Phys.Rev.Lett.99,026601(2007).
21.A.F.Otte et al .,Nat.Phys.4,847(2008).
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(2001).
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6,2519(1994).
24.D.Goldhaber-Gordon et al .,Phys.Rev.Lett.81,5225
(1998).
25.J.J.Parks,thesis,Cornell University,Ithaca,NY
(2009).
26.N.Roch,S.Florens,T.A.Costi,W.Wernsdorfer,
F.Balestro,Phys.Rev.Lett.103,197202(2009).
27.G.Christou,D.Gatteschi,D.N.Hendrickson,R.Sessoli,
MRS Bull.25,66(November 2000).
28.L.Bogani,W.Wernsdorfer,Nat.Mater.7,179(2008).29.We thank I.Cohen,M.Grobis,G.Hutchison,and
P.McEuen for discussions and K.Bolotin,J.Gro,F.Kuemmeth,and E.Tam for technical help.Rearch at Cornell was supported by the NSF through the Cornell Center for Materials Rearch,DMR-0605742,CHE-0403806,and u of the Cornell Nanofabrication Facility/National Nanotechnology Infrastructure Network.T.A.C.acknowledges supercomputer support by the John von Neumann Institute for Computing (Jülich).P.S.C.,A.A.A.,and C.A.B.were supported by Proyectos de Investigación Plurianuales 11220080101821of CONICET.
Supporting Online Material
www.sciencemag/cgi/content/full/328/5984/1370/DC1Materials and Methods SOM Text Figs.S1to S8Table S1References
11January 2010;accepted 29April 201010.1126/science.1186874
Nanoscale Tunable Reduction of
Graphene Oxide for Graphene Electronics
Zhongqing Wei,1*Debin Wang,2*Suenne Kim,2Soo-Young Kim,3,4Yike Hu,2Michael K.Yakes,1Arnaldo R.Laracuente,1Zhenting Dai,5Seth R.Marder,3Claire Berger,2,6William P.King,5Walter A.de Heer,2Paul E.Sheehan,1†Elisa Riedo 2†
The reduced form of graphene oxide (GO)is an attractive alternative to graphene for
producing large-scale flexible conductors and for creating devices that require an electronic
gap.We report on a means to tune the topographical and electrical properties of reduced GO (rGO)with nanoscopic resolution by local thermal reduction of GO with a heated atomic force microscope tip.The rGO regions are up to four orders of magnitude more conductive than pristine GO.No sign of tip wear or sample tearing was obrved.Variably conductive nanoribbons with dimensions down to 12nanometers could be produced in oxidized epitaxial graphene films in a single step that is clean,rapid,and reliable.G
ditraphene ’s high electronic mobility (1)has been harnesd in devices such as tran-sistors operating at gigahertz frequency
(2);however,the zero band gap of graphene leads to high leakage currents in many applica-tions.Another interesting material for a range of applications is graphene oxide (GO)(3,4),which exhibits a transport gap greater than 0.5eV at room temperature and becomes a miconductor or mimetal as it is reduced back toward graphene (5,6).Reduced GO (rGO)rembles graphene but with some residual oxygen and structural de-fects,yielding a conductivity that is comparable to that of doped conductive polymers (7)and 33,000times higher than that of doped hydrogenated
amorphous Si (8).Reduced GO can also be ud in highly nsitive gas nsors (9)and mechanical resonators with figures of merit surpassing tho of graphene resonators (10).
We prent a tip-bad thermochemical nano-lithography method to control the extent of re-duction of GO and pattern nanoscale regions of rGO within a GO sheet at speeds of veral m m/s.The relative increa in conductivity is as high as four orders of magnitude.GO was converted to rGO with a 100%yield in dozens of structures patterned on random locations in the GO film.Reduced GO patterns range from ribbons 12nm in width (full width at half maximum,FWHM)up to 20m m.No sign of tip wear or sample tearing
was obrved,indicating that the “carbon skele-ton ”is continuous across the GO/rGO junction.Therm
ochemical nanolithography (TCNL)with heated probe tips can localize thermally induced chemical reactions on a surface (11–14)or de-posit material (15–17).Similar heated tips have also been ud to mechanically modify a polymer film (18).We performed TCNL by using a heated atomic force microscope (AFM)probe tip to reduce lected regions of both single layers of isolated GO and large-area GO films formed by on-chip oxidation of epitaxial graphene (GO epi )grown on SiC.
Exposure of GO to strong reducing agents like hydrazine results in an incread electrical conductivity by three to four orders of magnitude (19).Thermal reduction of GO occurs already at moderate temperature (100°to 250°C)and enables tuning the gap in graphene oxide (6),as dem-onstrated in its current-voltage (I -V )character-istics.Recent studies have shown that annealing
1
Chemistry Division,U.S.Naval Rearch Laboratory,Code 6177,Washington,DC 20375,USA.2School of Physics,Georgia Institute of Technology,Atlanta,GA 30332,USA.3
School of Chemistry and Biochemistry,Georgia Institute of Technology,Atlanta,GA 30332,USA.4School of Chemical Engineering and Materials Science,Chung Ang University,Seoul 156-756,Republic of Korea.5Department of Me-chanical Science and Engineering,University of Illinoi
s Urbana-Champaign,Urbana,IL 61801,USA.6CNRS-Institut Néel,BP166,38042Grenoble Cedex 9,France.
*The authors contributed equally to this work.
†To whom correspondence should be addresd.E-mail:paul.sheehan@nrl.navy.mil (P.E.S.);elisa.riedo@physics.gatech.edu (E.R.)
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GO at 450°C or above is equivalent to chemical reduction via hydrazine monohydrate at 80°C follow
ed by heating at 200°C (20).We verified TCNL reduction of GO by friction force micros-copy (FFM),conductive AFM (CAFM),Raman spectroscopy,Kelvin probe force microscopy (KPFM),and ultrahigh-vacuum (UHV)elec-tronic transport measurements using a two-and four-point probe scanning tunneling microscope (STM)[e details in the supporting online ma-terial (SOM)(21)].
Arbitrary rGO features such as a cross (Fig.1)or squares (Fig.2)are reliably obtained by scan-ning the heated AFM tip over isolated GO flakes on a SiO x /Si substrate.The thermal reduction decreas the 9.5T 1.9Åheight of the sheet by 2to 5Å,as obtained from the topography image (Fig.1and fig.S6).Two effects could lead to height reduction.One is the loss of oxygen-rich functional groups from the GO flake surface.Given that scanning an unheated tip does not re-sult in height changes,this loss is primarily caud by intrinsic chemical conversion rather than me-chanical removal.It is not possible,however,to rule out tribochemical effects at elevated temper-atures.Second,the conversion of GO ’s sp 3carbon bonds into sp 2carbon bonds will flatten the ma-terial becau the sp 3carbon bonds in GO ripple the carbon skeleton,thereby increasing the sheet thickness (22–24).
Friction measurements show that variable re-duction of GO could be achieved by controlling the temperature of the AFM tip.Graphene has a low friction coefficient (25),whereas oxides typ-ically hav
wor
e higher friction coefficients.Thermal reduction should also reduce friction as the high-friction GO is replaced with lower-friction graphene.Figure 2shows the strong correlation between the cantilever temperature during TCNL process-ing and the lateral force on a room-temperature tip scanned over previously reduced squares.Whereas the cantilever temperature can be pre-cily determined,the contact temperature must be modeled (e discussion in the SOM)(21);the reported temperatures are the cantilever temper-atures.Reduction begins at or above 130°C,which is comparable to the results of Wu et al .and Mattevi et al .,who showed that reduction starts at 100°C (6,20),presumably after the desorption of adventitious water.Higher temperatures in-cread the rate of reduction,as shown by the roughly linear decrea in relative friction with temperature.
Although isolated GO flakes are suited for basic studies,further technological development requires extended films of GO.Large-area GO epi films (>15mm 2)were obtained by oxidizing mul-tilayer epitaxial graphene (EG)grown on the car-bon face of SiC [e material details in the SOM (21)].The oxidized films consist of multiple high-quality GO epi layers that completely cover the SiC surface.AFM images show no tearing in the GO epi films,indicating that they maintain their structural integrity when expod to the harsh oxi-dation conditions.Figures 3and 4show the re-sults obtained by performing TCNL on GO epi films with different thickness,as determined by AFM by scratching
away GO epi from the SiC substrate [e the SOM (21)].Figure 3prents a zigzag rGO epi nanoribbon written with a single line scan at T heater ~1060°C on GO epi .Figure 3A is an im-age of the current measured between a conductive platinum AFM tip and each point of the surface,showing no current on the GO surface and a cur-rent enhancement of about 100pA in the rGO epi nanoribbons.The current values are consistent with the prence of 12-nm-wide and veral-nanometers-thick rGO epi nanoribbons,prent-ing a vanishingly small Schottky barrier,and a resistive SiC substrate (resistivity of about 105ohm·cm).For a 25-nm-thick GO film locally heated by a tip at 1000°C,heat flow through the layers might reduce most of the GO un-derneath the tip and leave only a few layers of
GO at the SiC interface,as shown in the SOM (21).The topographical image (Fig.3B and black graph in Fig.3C)indicates that the reduction produces a shallow indentation of 1nm who origin has been previously discusd for the iso-lated GO sheets.
We further investigated the electrical proper-ties of the locally reduced GO epi structures using KPFM and four-point probe transport measure-ments in a UHV Omicron Nanoprobe system [e details in the SOM (21)].The sheet resistance,R sheet ,of 20m m by 20m m squares of TCNL rGO epi decread with increasing temperature ud for the TCNL local reduction,up to four orders of ma
gnitude lower than the resistance of the orig-inal GO epi (427T 11megohm)(6).The same decrea of the in-plane resistivity was obrved for extended films of rGO epi produced by over-night heating of GO epi in a furnace at 600°C (18T
Fig.1.Local thermal reduction of a single-layered graphene oxide flake.(A )Topography of a cross shape of reduced GO formed after an AFM tip heats the contact to 330°C scanned across the GO sheet at 2m m/s.(B )The averaged profile of the trench outlined in (A)shows that the width (FWHM)of the line can be as narrow as 25
nm.
Fig.2.The rate of thermal re-duction depends on the tip temper-ature.The plot shows the decrea in lateral force on an AFM tip at room temperature as it scans over veral squares previously reduced by TCNL at different temperatures.The int is a room-temperature fric-tion image of the GO sheet on which a heated tip was previously rastered twice over six square areas,at a speed of 4m m/s.In square 1,the tip was heated during TCNL to T heater ~100°C,yielding no apparent reduction,where-as at temperatures T heater >150°C the rastered areas (squares 2to 6)were thermally reduced.Reduced GO,which like bulk graphite behaves as a lu-bricant,shows lower friction than
the original GO.Higher temperatures accelerate the thermal reduction of GO and thereby more rapidly lower
friction.
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10kilohm).Furthermore,R sheet and the shape of the I -V characteristics could be varied by changing the temperature of the AFM probe (in Fig.4A,R sheet =9174kilohm and 30kilohm for low and high temperature,respectively).Kelvin probe mea-surements show that TCNL rGO epi displays a con-tact potential change of 168T 54mV in respect to GO epi ,similar to that of bulk reduced rGO (188T 96mV).The prence of residual oxygen and structural disorder led to the large difference in con-ductivity between epitaxial graphene and rGO epi or TCNL-rGO epi .
We also analyzed an isolated TCNL-rGO epi nanoribbon (Fig.4B)with a length of 25m m and a width of 100nm,as measured by AFM.We acquired I -V data by placing conductive tips on top of two micrometer-sized squares of rGO epi fabricated in situ by an electron beam at each end of the nanor
ibbon.Two-point trans-port measurements indicated a resistance larger than 2gigohm when the tips were placed at an arbitrary position on the GO surface (very large barrier at the contact)and a drop in resistance from 120megohm (between the two squares with no nanoribbon)to 20megohm (between the two squares connected by the nanoribbon).The transport changed from insulating to me-tallic (i.e.,linear I -V curves)in the prence of the TCNL-rGO epi nanoribbon between the squares (Fig.4B).By using the relation R sheet =(R ribbon ·w ·t ribbon )/(L ·t sheet )(26)and assuming a 13-nm-thick nanoribbon,we obtain a sheet resist-ance of 65kilohm,in good agreement with the measurements reported in Fig.4A for microscopic squares of TCNL-rGO epi .
TCNL does not require any solvents or litho-graphic resists that could contaminate the sample.This is especially important becau the elec-tronic properties of graphene vary strongly with surface doping.The strategy of variably reduc-ing extended GO films is a general one that could be implemented in multiple ways depend-ing on the application.The manufacture of gra-phene nanoelectronics could be achieved by
using arrays of heated probe tips (18).Indepen-dently addresd heated probe tips could alter-nately read or write nanostructures on a surface and in large arrays could address wafer-scale areas at high speed.A nano-embossing ap-proach might also achieve local GO reduction,provided that the im
auctionprint template would offer nanometer-scale control of the reducing tem-perature field.
References and Notes
1.C.Berger et al .,Science 312,1191(2006).
2.J.Kedzierski et al .,IEEE Trans.Electron.Dev.55,2078(2008).
3.D.A.Dikin et al .,Nature 448,457(2007).
4.S.Park,R.S.Ruoff,Nat.Nanotechnol.4,217(2009).
5.G.Eda,C.Mattevi,H.Yamaguchi,H.Kim,M.Chhowalla,J.Phys.Chem.C 113,15768(2009).
6.X.S.Wu et al .,Phys.Rev.Lett.101,026801(2008).
7.A.J.Epstein,in Conductive Polymers and Plastics:In Industrial Applications ,L.Rupprecht,Ed.(William Andrew,Norwich,NY,1999),pp.1–4.
8.C.E.Parman,N.E.Israeloff,J.Kakalios,Phys.Rev.B 47,12578(1993).
9.J.T.Robinson,F.K.Perkins,E.S.Snow,Z.Q.Wei,P.E.Sheehan,Nano Lett.8,3137(2008).
10.J.T.Robinson et al .,Nano Lett.8,3441(2008).
11.D.Wang et al .,Appl.Phys.Lett.95,233108(2009).12.D.Wang et al .,Adv.Funct.Mater.19,3696
(2009).
13.R.Szoszkiewicz et al .,Nano Lett.7,1064(2007).14.D.Wang et al .,Appl.Phys.Lett.91,243104(2007).15.P.E.Sheehan,L.J.Whitman,W.P.King,B.A.Nelson,
Appl.Phys.Lett.85,1589(2004).
but you16.M.Yang,P.E.Sheehan,W.P.King,L.J.Whitman,J.Am.
Chem.Soc.128,6774(2006).
17.B.A.Nelson,W.P.King,A.R.Laracuente,P.E.Sheehan,
L.J.Whitman,Appl.Phys.Lett.88,033104(2006).18.P.Vettiger et al .,IEEE Trans.NanoTechnol.1,39
(2002).
19.C.Gómez-Navarro et al .,Nano Lett.7,3499(2007).20.C.Mattevi et al .,Adv.Funct.Mater.19,2577(2009).21.See supporting material available on Science Online.22.G.Eda,G.Fanchini,M.Chhowalla,Nat.Nanotechnol.3,
270(2008).
23.H.C.Schniepp et al .,J.Phys.Chem.B 110,8535
(2006).
24.J.C.Meyer et al .,Nature 446,60(2007).
25.T.Filleter et al .,Phys.Rev.Lett.102,086102
(2009).
26.The thickness of the ribbon,t ribbon ,and the thickness of
the sheet,t sheet ,have been inferred from the thickness of the corresponding GO films as reported in the SOM.27.This work has been supported by the National Science
Foundation (CMDITR program DMR 0120967,Materials Rearch Science and Engineering Center program DMR
Fig.4.Four-point and two-point transport mea-surements.(A )I -V curves obtained by four-point transport measurements ofTCNL-reducedgraphene oxide squares reduced at low temperature (Low T ,T heater ~600°C),TCNL-reduced graphene oxide squares reduced at high temperature (High T ,T heater ~1200°C),and furnace-reduced graphene oxide at 600°C in vacuum.(B )I -V curves obtained by two-point transport mea-surements of current be-tween two rGO epi squares with no nanoribbons in be-tween (left curve),and be-tween two rGO epi squares with a nanoribbon in be-tween (rightplaybadminton
curve).
Fig.3.Local thermal reduction of a GO epi film:current and topo-graphical images.(A )Room-temperature AFM current image (taken with a bias voltage of 2.5V between tip and substrate)of a zigzag-shaped nanoribbon fabricated by TCNL on GO epi at T heater ~1060°C with a linear speed of 0.2m m s −1and a load of 120nN.(B )Corresponding topography image taken simul-taneously with (A).(C )Averaged profiles of current and height of the cross ctions that are indicated as dashed lines in (A)and
(B).www.sciencemag SCIENCE
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0820382,and DMR-0706031),U.S.Department of Energy (DE-FG02-06ER46293and PECASE),the Institute for Nanoscience at Naval Rearch Laboratory (NRL),the Office of Naval Rearch of the United States,the Defen Advanced Rearch Projects Agency (DARPA)Tip-Bad Nanomanufacturing program,and Georgia Institute of Technology (Georgia Tech Rearch
Foundation,COE Cutting Edge Rearch Award,and COPE
fellowship).This rearch was performed while Z.W.held a National Rearch Council Rearch Associateship Award at Naval Rearch Laboratory of the United States.Z.W.thanks H.Qi at NRL for providing doped silicon wafers,evaporating gold layers on mica,and taking scanning electron micrograph images of GO sheets on gold and silicon substrates.E.R.thanks T.-D.Li and M.Lucas for support with CAFM.
Supporting Online Material
www.sciencemag/cgi/content/full/328/5984/1373/DC1Materials and Methods Figs.S1to S8
References and Notes
9February 2010;accepted 29April 201010.1126/science.1188119
Quaternary Ammonium (Hypo)iodite Catalysis for Enantiolective Oxidative Cycloetherification
Muhammet Uyanik,1Hiroaki Okamoto,1Takeshi Yasui,1Kazuaki Ishihara 1,2*
It is desirable to minimize the u of rare or toxic metals for oxidative reactions in the synthesis of pharmaceutical products.Hypervalent iodine compounds are environmentally benign alternatives,but their catalytic u,particularly for asymmetric transformations,has
been quite limited.We report here an enantiolective oxidative cycloetherification of ketophenols to 2-acyl-2,3-dihydrobenzofuran derivatives,catalyzed by in situ –generated chiral quaternary ammonium (hypo)iodite salts,with hydrogen peroxide as an environmentally benign oxidant.The optically active 2-acyl 2,3-dihydrobenzofuran skeleton is a key structure in veral biologically active compounds.
O
ver the past two decades,hypervalent iodine compounds have been increasingly explored as environmentally benign oxi-dation reagents in place of rare or toxic heavy metal oxidants (1,2).However,their stoichio-metric u has been limited becau of potentially explosive shock-nsitivity and/or poor solubility in common organic solvents (1,2).Thus,the de-velopment of hypervalent iodine-catalyzed re-actions using more convenient stoichiometric co-oxidants is needed (3,4).Harnessing chiral hypervalent iodine compounds for enantiolec-tive oxidative coupling has proven a particular challenge in asymmetric catalysis.There are v-eral examples of catalysis by in situ –generated chiral aryl-l 3-or aryl-l 5-iodane (5)with meta -chloroperbenzoic acid (m -CPBA)as a co-oxidant (Fig.1A,left)(6–9);the include Quideau et al .’s enantiolective hydroxylative dearomatization of phenols (6),Altermann et al .’s enantiolective a -oxysulfonylation of ketones (7),and Dohi et al .’s and our independently reported enantiolective oxidative spirolactonizations of 1-napthol deriv-atives (8,9).In contrast,no strong examples have emerged of asymmetric catalysis using chiral cat-ions paired with inorganic iodine-derived oxo-acids,such as hypoiodous acid [IOH,I(I)],iodous acid [O=IOH,I(III)],iodic acid [(O=)2IOH,I(V)],and periodic acid [(O=)3IOH,I(VII)].
We report here an implementation of this strategy,using the atom-economical hydrogen per-oxide as a mild stoichiometric oxidant to activate
catalytic ion pairs of chiral quaternary ammonium iodide (Fig.1A,right)(10,11).Specifically,we tar-geted enantiolective oxidative cycloetherification of ketophenols to 2-acyl-2,3-dihydrobenzofuran derivatives,using a C 2-symmetric chiral binaphthyl-bad quaternary ammonium (hypo)iodite cata-lyst generated in situ by reaction with hydrogen
peroxide (Fig.1B).The chiral 2-substituted 2,3-dihydrobenzofuran skeleton is a key structure in veral biologically active compounds of me-dicinal interest (12–19).Earlier preparations of optically active 2-alkenyl-2,3-dihydrobenzofuran derivatives have relied on transition metal ca-talysis (20–23).
The catalytic or stoichiometric oxidation of 3-(2-hydroxyphenyl)-1-phenylpropan-1-one (1)with phenyl-l 3-iodanes gave a complex mixture,and the desired (2,3-dihydrobenzofuran-2-yl)(phenyl)methanone (2)was not detected (Fig.2A,entries 1and 2).In sharp contrast,and to our delight,the oxidation of 1with two equivalents of hydrogen peroxide [30weight percent (wt %)in water]in the prence of 10mole percent (mol %)of tetrabutylammonium iodide (Bu 4NI)in acetonitrile at room temperature gave 2in 87%yield (Fig.2A,entry 3)(24).Furthermore,the oxidation of 1was much faster in diethyl ether (Et 2O),tetrahydrofuran (THF),or ethyl acetate (EtOAc)(Fig.2A,entry 4,and table S1).Notably,the oxidation of 1did not occur on substitution of tetrabutylammonium bromide or chloride for Bu 4NI.Excellent chemolectivity was obrved
1
Graduate School of Engineering,Nagoya University,Furo-cho,Chikusa,Nagoya,464-8603,Japan.2Core Rearch for Evo-lutional Science and Technology,Japan Science and Technology Agency,Furo-cho,Chickusa,Nagoya,463-8603,Japan.*To whom correspondence should be addresd.E-mail:
ishihara@cc.nagoya-u.ac.jp
Fig.1.(A )(Left)Known in situ –generated aryl-l 3-or aryl-l 5-iodane catalysis.(Right)In situ –generated hypoiodite(I)or iodite(III)catalysis.(B )Design of inorganic iodide precatalyst paired with a chiral quaternary ammonium counter ion for the enantiolective oxidative cycloetherification of ketophenols to 2-acyl-2,3-dihydrobenzofuran derivatives.Ar,aryl;L,ligand;M +,metal or onium cation,R,alkyl or aryl group.Symbols (Ar and L)marked with asterisks reprent chiral groups.
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Nanoscale Tunable Reduction of Graphene Oxide for Graphene
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