scanning tunneling spectroscopy observations of the...

Ž .Applied Surface Science 151 1999 262–270www.elsevier.nlrlocaterapsusc

Scanning tunneling spectroscopy observations of the Coulombstaircase at room temperature from the dioctyldithiophosphate

ž /sodium salt molecules deposited on the 0001 graphite surface

Z. Klusek ), M. Luczak, W. OlejniczakDepartment of Solid State Physics, UniÕersity of Lodz, Pomorska 149r153, Łodz 90-236, Poland´ ´

Received 16 March 1999; accepted 16 June 1999

Abstract

Ž . Ž .In our contribution, we discuss the growth of the NaDDP dioctyldithiophosphate sodium salt molecules on the 0001graphite substrate from solution. We observe rod-like structures and smooth terraces. The scanning tunneling spectroscopyŽ .STS measurements on narrow rods show a distinct Coulomb gap and step-like structures, where steps are more pronouncedat the occupied part of the spectra. To explain the observed phenomenon, we consider a double junction model, in which

Ž .electrons tunnel through two barriers. One is formed between the scanning tunneling microscopy STM tip and themolecule. The other one is the space interval at the moleculergraphite interface. Moreover, we observe the existence of the

Ž .negative differential resistance NDR regions on the IrV characteristics. We consider the resonant tunneling via modifiedelectronic states of the combined moleculergraphite system to explain observed phenomenon. q 1999 Elsevier Science B.V.All rights reserved.

PACS: 61.16.Ch; 81.10.Dn; 68.35.Bs; 73.40.Gk; 73.61.y r

Keywords: Scanning tunneling microscopy; Scanning tunneling spectroscopy; Molecules; Coulomb blockade; Resonant states

1. Introduction

The Coulomb blockade is observed by scanningtunneling microscopyrscanning tunneling spec-

Ž .troscopy STMrSTS techniques in different sys-w xtems 1–7 . In the STM configuration, a tip is placed

w x w xabove a metal cluster 3 or a molecule 5,6 de-

) Corresponding author. Tel.: q48-42-6355704; fax: q48-42-6790030; E-mail: [email protected]

posited on an insulating layer. The latter is grown onconductive material. The vacuum gap between a tip

Ž .and a cluster or a molecule and an insulating layerform a well-defined double junction system. Re-cently, different organic molecules have extensively

w xbeen studied as insulating layers 3,4 .A double junction system results in the suppres-

sion of tunneling current near zero bias voltage —w xCoulomb gap 1,2 . The gap has the voltage width of

DVser2C, where C denotes the total capacitanceof the island to its environment and e the charge of

0169-4332r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.Ž .PII: S0169-4332 99 00281-0

( )Z. Klusek et al.rApplied Surface Science 151 1999 262–270 263

the electron. In addition to the Coulomb gap, astrongly asymmetric junction system exhibits a seriesof steps on the currentrvoltage characteristic. The

w xphenomenon is called the Coulomb staircase 1,2 .The steps have the voltage width of DVserC and

Ž .the current rise of D IserRC see inset in Fig. 8 ,where R denotes the total junction resistance. Theonly exception is the first current step, where D Iser2 RC. The steps are observed for the temperatures

2 Žof kT-e r2C k — Boltzmann’s constant, T —.temperature .

The question is whether a molecule adsorbed on aclean surface without any insulating layer can betreated as a Coulomb blockade structure or not. Inthe configuration, one barrier is formed between anSTM tip and a molecule and the other one is a spaceinterval at a moleculersubstrate interface. Recently,the model with naturally provided potential wall atthe interface has been considered to explain oscilla-tions of differential conductance measured over indi-

Ž .vidual Ag clusters on the clean GaAs 110 surfacew x7 .

The goal of this study is to obtain tunnelingŽspectra of the NaDDP dioctyldithiophosphate.sodium salt, see insets in Fig. 6 molecules deposited

Ž .from solution on the 0001 basal plane of highlyŽ .oriented pyrolytic graphite HOPG substrate. If so,

they let us determine whether the Coulomb blockadeeffect takes place or not. Then we can understandbetter the transport of electrons through the NaDDPmolecules and interpret the spectra obtained fromother molecular systems.

2. Experimental

In our experiments, we use two types of graphiteŽ .surfaces. One is the pure 0001 basal plane of

Ž .HOPG, and the other one is the 0001 basal plane ofHOPG with thermally-etched circular monolayer pits.In this case, the pits or pit edges are used as nanome-ter surface containers or anchoring sites for theNaDDP molecules.

We prepare a new surface of pure HOPG bycleavage with adhesive tape. In the case of thermallyetched HOPG sample, the cleavage procedure isapplied just before thermal oxidation. The oxidationprocess is carried out at the temperature of 8008C.

The time of heating is 30 min. After the treatment,the sample is cooled in a natural way in ambient air.The experimental details are presented elsewherew x8,9 . Finally, the pure graphite sample and the sam-ple with etched pits are immersed into 1 mM solu-tion of NaDDP in ethanol. The adsorption time is 24h. Then the samples are dried in ambient air andstudied by STM.

The STMrSTS results are obtained by apparatusw xwith spectroscopic facilities 10 . The studies are

performed in air, in the constant current mode withthe typical tunneling current set point of 0.2–1 nAand the sample bias of 1 V. The tips are obtained bymechanical cut from the Pt –Ir alloy wires. The90 10

IrV characteristics are recorded in the current imag-Ž .ing tunneling spectroscopy CITS mode.

3. STM results

In Fig. 1, a typical STM image of etched pits onan HOPG sample treated at 8008C in air for 30 minis shown. The detailed STMrSTSrCITS studies ofthermally oxidized graphite surface are presented

w xelsewhere 8,9 .Fig. 2 shows the STM image of the NaDDP

molecules adsorbed from solution on the thermallyetched HOPG substrate. We expect that the deposi-tion process is initiated at the pit edges. The pre-sented image shows bright structures with nearlycircular symmetry. This is especially seen for theobject shaped like a ring in the center of the image.In our opinion, it proves that the growth of molecularlayers starts at surface defects. At the initial stage ofthe growth, the pit symmetry is retained.

Apart from the typical topography presented inFig. 2, islands of roughly 10–20 nm in height andseparated by flat graphite regions can be seen aswell. The extension of the islands and the distancebetween them differ slightly in different experiments.It seems that the island growth is initiated by the pitedges. Then the islands can be treated as successivestages of the molecular crystal growth.

The appearance of the two-dimensional molecularlayer or thin molecular film is also occasionallyobserved. Then they surround the islands. An atomicstructure of the HOPG basal plane can be seen awayfrom these regions. A detailed structure of the

( )Z. Klusek et al.rApplied Surface Science 151 1999 262–270264

Fig. 1. The 600 nm=600 nm STM image of the thermally etched graphite at 8008C for 30 min. The STM image is recorded in constantcurrent mode at sample bias of 0.2 V. The tunneling current set point is 1 nA.

molecular layerrfilm can only be observed in con-stant height mode as presented in Fig. 3. The topog-raphy consists of several bright narrow rods whichare perpendicular to scan direction and whose widthvaries from 1.5 nm to 6 nm.

As it has been found during the experiments, theconstant current images give worse topographicalresults. The images taken at the same sample biasbut at different stabilization current values in the

Ž .range of 0.2–1 nA different tip-sample distance aredifferent. The image corrugation depends on the

applied stabilization current in a roughly linear fash-ion. With an increase of current, the maxima becomemore distinct. We suppose that elastic effects be-tween the tip and the sample may play some rolehere. The images recorded at the same stabilizationcurrent but at different bias, voltages vary. This isbecause not only the tip-sample distance is changedŽ .constant current mode but also different electronicstates participate in tunneling current.

It is difficult to suggest a model which arrangesmolecules on the graphite substrate since the ob-

Fig. 2. The 143 nm=143 nm STM image of the NaDDP molecules adsorbed from solution onto the thermally etched HOPG substrate. Thedeposition process is initiated at the pit edges. The STM image is recorded in constant current mode at sample bias of 1 V. The tunnelingcurrent set point is 0.2 nA.


Fig. 3. The 28.8 nm=28.8 nm STM image of the NaDDP molecules adsorbed from solution onto the thermally etched HOPG substrate.The narrow rod structure is presented. The STM image is recorded in constant height mode at sample bias of 1 V. The tunneling current setpoint is 0.2 nA.

tained topographical results are strongly affected bythe electronic states of the moleculergraphite sys-

tem. This problem should be the subject of furtherexperimental and theoretical studies.

Fig. 4. The 136 nm=136 nm STM image of the NaDDP molecules adsorbed from solution onto the pure HOPG substrate. The wide rodstructure is presented. The STM image is recorded in constant current mode at sample bias of 1 V. The tunneling current set point is 0.2 nA.


STM investigations on graphite without pits givedifferent topographies. In Fig. 4, we see structuresshaped like rods which are larger than those pre-sented in Fig. 3. It is clear that here we observethree-dimensional molecular crystal on the HOPGsubstrate. High-resolution images of the wide rodsand the regions between them are obtained in con-stant current and height modes. However, no recog-nizable periodic structure can be resolved.

Apart from the structures shaped like rods, flatterraces formed from the NaDDP molecules areobserved as presented in Fig. 5. In this case, awell-defined three-dimensional molecular crystal isdeveloped. It is worth pointing out that this imageresembles the one obtained for the thermally-

Ž .evaporated gold on the heated HOPG 0001 sub-Ž .strate, where well-defined Au 111 terraces are ob-

w xserved 11 . As in the case of wide rods, no recogniz-able periodic structure can be resolved by STM onthe flat molecular terraces.

In conclusion, the obtained STM results show thaton pure HOPG, the deposition process leads to theappearance of a well-defined three-dimensionalmolecular crystal. On the graphite defected by circu-lar pits, we observe the growth of islands initiated bypit edges. Occasionally, the regions with the two-di-mensional molecular crystal or thin molecular layercan be found. It should be emphasized that the

growth process of the NaDDP molecules from solu-Ž .tion on the HOPG 0001 basal plane requires further

studies.

4. STS results

The next stage of our studies aims at measuringŽ .the tunneling spectra IrV in the regions in which

three and two-dimensional molecular crystals areobserved.

In Fig. 6, the typical IrV data, which are recordedover the flat terraces presented in Fig. 5, are shown.The curves are recorded at different points and showthe same shape. The curves obtained over the widerods are similar to those presented in Fig. 6. Thegeneral shape of these curves resemble the tunnelingspectroscopy obtained for the pure unaffectedgraphite. The curves presented in Fig. 6 show a littleasymmetric shape, the tunneling current being higherfor negative polarization of the sample than for apositive voltage of the same value. This type of

w xasymmetry is typical for the pure graphite 8,12–14 .Lastly, the IrV spectra recorded on the three-di-

mensional NaDDP molecular crystal reflect metallicŽ .or semimetallic graphite behavior. In the presented

curves, the suppression of the tunneling current atzero bias voltage, which suggests the existence of the

Fig. 5. The 617 nm=617 nm STM image of the NaDDP molecules adsorbed from solution onto the pure HOPG substrate, wherethree-dimensional molecular crystal is developed. The image is recorded in constant current mode at sample bias of 1 V. The tunnelingcurrent set point is 0.2 nA.


Fig. 6. The Ir V spectra recorded at two different points onterraces presented in Fig. 5. The insets show the structure of theNaDDP molecule.

Coulomb blockade at room temperature, is not ob-served. The lack of the Coulomb gap does notdepend on the tunneling current set point which fixesthe tip-surface distance.

The final stage of our studies is to measure thetunneling spectra on the NaDDP regions presented in

Ž . Ž .Fig. 3. In Fig. 7, the curves denoted as a , b andŽ .c are recorded at different tip-sample distances.They are recorded at the same bias voltage of 0.9 V,while the tunneling current set point is changed in

Ž .the range of 0.6–1 nA. The a curve is recordedŽ .close to the surface and the c curve far from theŽ .surface. The curve denoted as b is recorded at the

Ž . Ž .intermediate distance. The a and b curves shownearly the same shape. Here we observe the regionswhere tunneling current decreases with an increaseof bias voltage, which suggests the presence of anegative slope in the IrV curves. This phenomenon

Ž .is called negative differential resistance NDRw x15,16 . In our interpretation, the appearance of NDRcan be explained by the resonant tunneling via modi-fied electronic states of the combined NaDDPr

Ž .HOPG 0001 system. It is worth recalling thatSTMrSTS make it possible to detect NDR in cur-rentrvoltage characteristics for the specific tip-sam-ple configurations. The configurations include con-volution of the sample electronic local density of

Ž .states LDOS with a contamination-induced peak inthe tip LDOS, charging of electron traps near thebarrier, and resonant tunneling in double barrier well

w xstructures 15,16 .First of all, we exclude convolution of the sample

LDOS with the contamination-induced peak in thetip LDOS as an origin of the observed NDR regions.This is because the observed NDR appears onlywhen the characteristics are recorded over the nar-row rods. The observed structures do not appearwhen IrV curves are collected over the HOPGplane away from the narrow rod edges. It means thatthe tip is not affected by the contamination-inducedpeak in the tip LDOS. In our opinion, the chargingof electron traps near the barrier should also beexcluded since this effect is rather typical formetal–insulator–semiconductor structures. Hence,only resonant tunneling via modified electronic states

Ž .of the combined NaDDPrHOPG 0001 system canbe considered an origin of the NDR phenomenon.

In this model, the adsorbed molecule is treated asŽ .a potential well with electronic states between the

w xtip and the graphite substrate 17,18 . The graphitemodulates the molecule states into resonances which

w xspread throughout the molecule gap 18 . When the

Fig. 7. The Ir V spectra recorded over a single narrow rod. TheŽ . Ž . Ž .three curves denoted as a , b and c are recorded at different

Ž .tip-sample distances. The curve a is recorded close to theŽ .surface, while the curve c far from the surface. The curve

Ž .denoted as b is recorded at the intermediate distance.


voltage bias is sufficient to adjust the quantum stateŽ .inside the well NaDDP molecule so that it is within

the range of energies for the tip conduction band,Ž .then the well NaDDP molecule is in resonance.

Then the current can flow onto the molecule and outto the graphite substrate. Otherwise, the current isblocked. It means that the system is out of reso-nance. As a result, the IrV characteristic with nearlyflat regions or NDR regions can be observed. On the

Ž . Ž .d IrdV and d IrdV r IrV curves the resonantstatesappear as pronounced peaks. When the system is outof resonance, the pronounced dips on the d IrdV andŽ . Ž .d IrdV r IrV characteristics are observed.

Ž .Now, we focus on the curve denoted as c . It isimmediately seen that the shape of this curve differs,particularly at the negative sample bias. The intensityand energetic positions of the NDR regions changesignificantly. Additionally, a pronounced step-likestructure is superimposed on the IrV curve. It meansthat apart from resonant tunneling, a new process isprobably observed.

In this curve, we observe distinct suppression ofthe tunneling current near the zero bias voltagewhich can be attributed to the Coulomb gap. Further-more, the observed step-like structure at negativebias voltage can be interpreted as the Coulomb stair-case. The width of the gap, determined by extrapolat-ing tangent at the first step to zero current, is 0.5 V.This can be related to the total junction capacitanceof Cser2DVs1.6=10y9 F. For such small ca-pacitance, the thermal energy kT at room tempera-ture is much smaller than the Coulomb chargingenergyof e2r2C. Taking into account the Coulomb gapvalue of er2Cs0.5 V and calculated dynamicalresistance of 1.8 GV, we find the first tunnelingcurrent rise of D Iser2 RCs0.3 nA. The mea-

Ž .sured value of D I from the c curve is 0.4 nA,which is consistent with the above.

The shape of the tunneling current characteristicrecorded on a double junction system depends on the

Žindividual junction resistances: R resistance of a1. Žtiprmolecule junction , R resistance of a2

.moleculersubstrate interface and on the capaci-Ž .tances: C capacitance of a tiprmolecule junction ,1

ŽC capacitance of a moleculersubstrate inter-2. Žface . The resistance of the first junction tiprmole-

.cule seems to be much larger than that of the secondŽ .junction moleculersubstrate interface , i.e., R 41

Ž .R . Then the total resistance RfR depends on2 1

tiprmolecule distance. In addition to the CoulombŽgap, the strongly asymmetric junction system R 41

.R , C 4C , C fC exhibits the Coulomb stair-2 1 2 1

case with the steps separated by DVserC voltagew xintervals 1,2,7 . If we assume that the nearly flat

regions on the IrV characteristic are due to theCoulomb charging, then their width can be calcu-lated as 1 V. The width of the flat region determinedfrom the first derivative of the tunneling currentd IrdV equals 0.8 V, which is consistent with thecalculated value. The presented analysis shows thatthe experimental results are in accord with theoreti-cal predictions for the Coulomb blockade effect.

One discrepancy to be observed easily is theasymmetry of current flow for negative and positivevoltage bias. The gap and the steps are pronouncedat the negative part of the spectra, i.e., for occupiedstates. The presence of the tunneling current asym-metry can be explained if we consider the effects ofthe discreteness of the energy spectrum in a molecule.When the molecule contains a relatively large clusterof metal atoms, the symmetric IrV curve with the

w xCoulomb gap and steps can be observed 19 . This isin agreement with the classical theory of the Coulombblockade oscillations which assumes the continuousenergy spectrum in the region confined by the tunnelbarriers. However, when the molecule contains arelatively small metal cluster or a single metal atomŽ .NaDDP , then instead of the classical theory of theCoulomb blockade, we should apply the theory whichincludes the effects of the discreteness of the energyspectrum. When the level discreteness plays a signif-icant role, there is no symmetry between additionand removal of electrons torfrom a molecule, whichleads to an asymmetric IrV curve. Furthermore, thechemical transformation of a molecule due to elec-tron transport should also be taken into account.Concluding, our spectroscopic results recorded onthe NaDDP molecular system can differ from thepredictions of the classical Coulomb blockade the-ory.

In order to confirm the conclusion that theCoulomb blockade effect takes place, additional ex-periments are performed. The energetic heterogene-ity on the surface and the influence of the tip-surface


distance on the Coulomb gap width are subjects ofour experiments.

In Fig. 8, the IrV curves recorded over differentpoints on the topography presented in Fig. 3 areshown. They are recorded at the same tunneling

Ž .conditions bias, tunneling current set point . Firstly,we observe that the suppression of the tunnelingcurrent at zero bias voltage is well visible. It sug-gests that the existence of the Coulomb gap is ob-served at room temperature. The width of the gap,determined by extrapolating tangent at the first stepto zero current, is 0.5 V. Secondly, it is easy to seethe nearly flat regions pronounced well on the tun-neling current characteristics. Their width equalsroughly 0.9 V. This observation is consistent withtheory of the Coulomb blockade effect. Moreover,the resonant tunneling via modified electronic states

Ž .of the combined NaDDPrHOPG 0001 system takesplace due to the existence of the NDR regions onIrV characteristics.

It should be emphasized that IrV data obtainedfrom different points over the narrow rods indicateenergetic heterogeneity. In particular, the curves withthe shape presented in Fig. 6 are frequently ob-served, in spite of the fact that these curves areobtained at the points close one to another. The

Fig. 8. The Ir V spectra recorded at three different points over asingle narrow rod. The inset shows ideal representation of thecurrentrvoltage characteristic expected from the Coulomb block-ade theory.

Ž .Fig. 9. Inverse capacitance vs. ln R . The linear behavior indi-cates a 1rd law for the capacitance.

recording of energetic heterogeneity seems to be animportant result of our investigations. The observeddifferences prove that narrow rods are not uniformeven in very small areas, so we can assume that theCoulomb blockade effect takes place only in theareas in which rods are of single molecular character.

The capacitance and the total tunneling resistanceof the tiprmolecule junction depend on the tunnelingcurrent set point which fixes the tip-sample distance.This is presented in Fig. 9, where the inverse capaci-

Ž .tance as a function of the ln R is shown. Since theŽ .ln R is proportional to the distance between the tip

and the surface, we find that the inverse capacitancechanges approximately linearly with the tip-sampledistance. Then, the local capacitance of the tiprmolecule can be described by a simple parallel-platecapacitor with the capacitance of CsASrd. Here, Sdenotes an effective area of the junction, d atiprsurface distance, and A a constant. The Coulombgap evolution is in accordance with other experimen-

w xtal results 20,21 .Our experiments confirm the conclusion that the

Coulomb blockade effect takes place in the NaDDPrŽ .HOPG 0001 system. Thus, we can adopt the model

in which moleculermolecules adsorbed on a cleansurface without any insulating layer form Coulomb

w xblockade structure 7 . However, more research needsto be done to understand the influence of local


surface defects on the obtained results. Further ex-periments are in progress.

5. Conclusions

By the use of STMrSTS methods, we have ob-served the growth of the NaDDP molecules on thepure and thermally etched graphite substrate fromsolution. In the case of pure graphite, the rod-likestructures and terraces are visible. On the thermallyetched graphite, the growth process is initiated at thepit edges and then the randomly distributed islandsare observed. At the island edges, the two-dimen-sional molecular layer is occasionally seen.

The STS results recorded in these areas showNDR regions on the IrV characteristics. The reso-nant tunneling via modified electronic states of thecombined moleculergraphite system explains theobserved phenomenon.

Moreover, at specific tunneling conditions, theIrV measurements show the distinct gap and thestep-like structure with the steps more pronounced atthe occupied part of the spectra. The data analysishas shown that the experimental results are in accor-dance with the theoretical predictions of the Coulombblockade effect on the double junction system. Inthis model, electrons tunnel through two barriers.One is formed between the STM tip and the moleculeand the other is the space interval at the moleculergraphite interface.

Acknowledgements

The work was supported by the Polish CommitteeŽfor Scientific Research Grant No. 7 T08C 04211

.and Grant No. 7 T08C 02010 . We wish to thank Dr.

L. Margielewski from the Chemical Technology De-partment of the University of Lodz for the prepara-tion of the NaDDP solution.

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