Scanning tunneling microscopy of layered molecular conductors

Michael A. Dvorak, Shulong Li, Michael Ward

Research output: Contribution to journalArticle

Abstract

Scanning tunneling microscopy (STM) of single crystals of semiconducting organic crystalline charge-transfer salts [R1(R2)NCH2CH2OCH2CH 2]+[TCNQ]2- (1a: R1 = C2H5, R2 = C2H5; 1b: R1 = CH3, R2 = C2H5; 1c: R1 = CH3, R2 = CH3, monoclinic; 1d: R1 = CH3, R2 = CH3, triclinic) and (Ph(H)NCH=CHCH=N(H)Ph)+(TCNQ)2- (TCNQ = tetracyanoquinodimethane) is described. The STM data reveal tunneling current features that compare favorably with the electronic properties and crystal structure of the salts. The local density of states (LDOS) exhibit orientation and corrugation consistent with the topography and electronic structure of dimerized TCNQ stacks contained in the molecular layers that define the exposed low-energy crystal planes. In the case of 1a, crystallographically inequivalent TCNQ layers that alternate through the crystal are revealed in STM data by sequential etching of the layers. The two dimorphs 1c and 1d reveal dramatically different tunneling current contrast due to the different molecular motifs of their respective crystal faces, as surmised from the crystal structure. While the lattice constants determined from the tunneling current contrast correspond favorably with the values expected based on single-crystal X-ray structures, in some cases there appears to be a slight compression of the surface layer. This behavior is consistent with weak interaction between the molecular layers of the charge-transfer salt, and the tendency of the molecular constituents in the surface layer to achieve a higher packing density in order to lower the crystal surface free energy. These studies reveal that STM can provide substantial insight into the structure of organic crystals, and can probe molecular-level structural and electronic features.

Original languageEnglish (US)
Pages (from-to)1386-1395
Number of pages10
JournalChemistry of Materials
Volume6
Issue number8
StatePublished - 1994

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Scanning tunneling microscopy
Crystals
Salts
Charge transfer
Crystal structure
Single crystals
Molecular Probes
Electronic properties
Topography
Free energy
Lattice constants
Electronic structure
Etching
tetracyanoquinodimethane
Crystalline materials
X rays

ASJC Scopus subject areas

  • Materials Science(all)
  • Materials Chemistry

Cite this

Scanning tunneling microscopy of layered molecular conductors. / Dvorak, Michael A.; Li, Shulong; Ward, Michael.

In: Chemistry of Materials, Vol. 6, No. 8, 1994, p. 1386-1395.

Research output: Contribution to journalArticle

Dvorak, Michael A. ; Li, Shulong ; Ward, Michael. / Scanning tunneling microscopy of layered molecular conductors. In: Chemistry of Materials. 1994 ; Vol. 6, No. 8. pp. 1386-1395.
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abstract = "Scanning tunneling microscopy (STM) of single crystals of semiconducting organic crystalline charge-transfer salts [R1(R2)NCH2CH2OCH2CH 2]+[TCNQ]2- (1a: R1 = C2H5, R2 = C2H5; 1b: R1 = CH3, R2 = C2H5; 1c: R1 = CH3, R2 = CH3, monoclinic; 1d: R1 = CH3, R2 = CH3, triclinic) and (Ph(H)NCH=CHCH=N(H)Ph)+(TCNQ)2- (TCNQ = tetracyanoquinodimethane) is described. The STM data reveal tunneling current features that compare favorably with the electronic properties and crystal structure of the salts. The local density of states (LDOS) exhibit orientation and corrugation consistent with the topography and electronic structure of dimerized TCNQ stacks contained in the molecular layers that define the exposed low-energy crystal planes. In the case of 1a, crystallographically inequivalent TCNQ layers that alternate through the crystal are revealed in STM data by sequential etching of the layers. The two dimorphs 1c and 1d reveal dramatically different tunneling current contrast due to the different molecular motifs of their respective crystal faces, as surmised from the crystal structure. While the lattice constants determined from the tunneling current contrast correspond favorably with the values expected based on single-crystal X-ray structures, in some cases there appears to be a slight compression of the surface layer. This behavior is consistent with weak interaction between the molecular layers of the charge-transfer salt, and the tendency of the molecular constituents in the surface layer to achieve a higher packing density in order to lower the crystal surface free energy. These studies reveal that STM can provide substantial insight into the structure of organic crystals, and can probe molecular-level structural and electronic features.",
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