Surface structure of ultrathin copolymer films of ferroelectric vinylidene fluoride (70%) with trifluoroethylene (30%) on graphite

The structure and local structural distortions, through the polarization manipulation, of crystalline films of ferroelectric vinylidene fluoride (70%) with trifluoroethylene (30%) [P(VDF-TrFE)] copolymer on graphite were studied by scanning tunneling microscopy (STM). A quasispiral twist in $\mathrm{C}\mathrm{C}$ bonds with rotations about the polymer chain axis was observed by high-resolution STM, indicating a surface relaxation of the strained copolymer films. Such a relaxation behavior appears to be linked to the observed local dipole rotations accompanied by the reversal of the local polarization with biasing the STM tip. A structure model is proposed based upon the observations.

Figure 1

(Color-online) (a) An atomically resolved STM image of the surface of P(VDF-TrFE 70:30) film on graphite at room temperature and (b) Ball structural model for a P(VDF-TrFE) chain in the all-trans configuration. Every bright spot represents a $\mathrm{C}{\mathrm{H}}_2\mathrm{C}{\mathrm{F}}_2$ or $\mathrm{C}\mathrm{H}\mathrm{F}\mathrm{C}{\mathrm{F}}_2$ monomer. The tip-bias voltage $V_{\text{bias}}$ and the tunneling current $I$ were $-0.36\mathrm{V}$ and $0.38\mathrm{nA}$, respectively. The image size is about $2.6\mathrm{nm}\times 2.6\mathrm{nm}$. The arrow shows the direction of monomer dipole moment. The surface unit cell is also indicated.

Figure 2

A proposed structure of P(VDF-TrFE) on graphite. Hollow circles represent substrate carbon atoms while filled circles represent copolymer monomers.

Figure 3

(Color online) (a) STM image of P(VDF-TrFE) on graphite at room temperature obtained with $V_{\text{bias}}=-0.10\mathrm{V}$ and $I=0.21\mathrm{nA}$. The image size is $4.4\mathrm{nm}\times 4.4\mathrm{nm}$. A fine intra-chain structure is shown in the image. The dashed black lines only guide the eye. (b) The same picture as Fig. 3a but with the grid showing the superstructure of the surface. The short solid blue or solid black line indicates two protrusions that belong to one monomer. The long dashed black line indicates the domain boundary. The inset shows the schematic intra-chain structure with four possible directions of monomers in the $(4\times 1)$ superlattice. The black arrows indicate directions of dipole moment. The dashed black lines only guide the eye.

Figure 4

(Color online) (a) The STM image on a P(VDF-TrFE) copolymer film at room temperature obtained by flipping the polarity of the tip bias voltage at different values. Image size is $4.4\mathrm{nm}\times 4.4\mathrm{nm}$. The polarity of the tip bias voltage was flipped from $0.57\mathrm{V}$ to $-0.57\mathrm{V}$ and then back to $0.57\mathrm{V}$ during measurement. The tunneling current was $0.6\mathrm{nA}$. The image of $\mathrm{C}{\mathrm{H}}_2∕\mathrm{C}\mathrm{H}\mathrm{F}$- or $\mathrm{C}{\mathrm{F}}_2$- clusters are indicated. (b) The “flip-reversal” model and (c) “cant-reversal” model. The two C-C bonds involved in the “twisting” distortion induced by STM are indicated in the circular shadows.