1. Introduction

The ability to control the position and orientation of nanorods (NRs) in a device is very interesting both from a scientific and a technological point of view [1–5] In particular, as NRs exhibit anisotropic absorption, and spontaneous and stimulated emission [6–9], aligning individual NRs to a preferred axis is attractive for many applications in photonics such as solar cells [10], light-emitting devices [11, 12], optical sensors, switches, etc [13–15].

Several techniques for aligning NRs have been explored such as: rod solubility in a binary solvent/non-solvent mixture [4], coffee stain evaporation dynamic [15–17], radial fluid flow with spin coating & drying lyotropic phase [1, 18], slow solvent evaporation at a liquid-solid-air interface [2, 12], Langmuir-Blodgett deposition [5], wet-chemical attachment of NRs & epitaxial growth [19], mechanical rubbing the rods [20] or external electric fields [21]. Among them, electric-field-driven deposition from colloidal suspensions have proven to be an efficient method for the controlled positioning and alignment of all kinds of anisotropic particles such as semiconductor [8, 13, 22–26] and gold [21, 27] NRs, nanowires [28] and carbon nanotubes [29].

For the alignment of NRs dispersed in an apolar liquid in an electric field, the permanent and the induced (dielectric) dipole moment are of the same order of magnitude according to the literature. Semiconductor NRs have typically a much higher dielectric constant than the apolar liquid and this results in a induced dipole moment that is proportional (but not parallel, due to the shape anisotropy) with the electric field. The permanent dipole moment of the NR is related to the non-centro-symmetric crystal structure of semiconductors like CdSe, but its value (order of hundreds of Debye) is orders of magnitude smaller than expected from the dipole moment of the unit cell of the crystal, due to electrostatic compensation. For small fields, the variation in absorbance and birefringence for permanent and induced dipole moments are all quadratic with the electric field. The asymmetry between on and off-switching in transient electric birefringence measurements of CdSe NRs by Li et al. [30] indicates that the permanent dipole moment is more important. Kamal et al. found in addition an increase in absorption anisotropy for illumination near the first exciton transition in CdSe/CdS NRs [31]. The net NR charge is usually zero, and as a result the NRs do not move to the electrodes in a DC electric field [8, 23, 30].

In contrast to the previously cited works, we use an AC field with high amplitude in which the alignment is considerable. This is the case when the difference in energy for parallel and perpendicular alignment $\frac{1}{2}({\alpha }_a-{\alpha }_b){\epsilon }_0{E}^2$ becomes larger than the thermal rotational energy $\frac{1}{2}kT$ [32]. In these formulas, E is the applied electric field in the liquid and ${\alpha }_a-{\alpha }_b$ is the difference between the parallel and perpendicular polarizability of the particles in the medium.

Using such strong AC fields Ahmed et al. already reported an effective alignment of gold NRs by drying a drop of a colloidal NRs suspension on a silicon wafer with platinum electrodes [27]. Similarly, Hu et al. achieved effective CdSe and CdTe NR alignment on Si3N4 membranes with electrodes through slow drying in N₂ atmosphere of a colloidal solution drop while applying a strong DC electric field, directly inside a small atomic force microscope chamber [23]. In these two papers good alignment is obtained, but the methods are based on the casting of single drops and subsequent drying of the solvent, which is not compatible with reproducible and homogeneous deposition on large substrates as required for large size applications such as solar cells or OLEDs.

In this work, we present a novel technique for the homogeneous deposition and strong alignment of CdSe/CdS NRs on a glass substrate patterned with transparent indium tin oxide (ITO) interdigitated electrodes with a spacing of a few micrometers. This fast and versatile method is based on applying an electric field over the electrodes during the dip-coating procedure and is compatible with large-scale processing on cheap and transparent substrates. The accumulation, alignment, and polarized fluorescence of the NRs as a function of the electrical field are investigated. An alignment with order parameter of 0.67 and a polarization ratio of 0.60 is obtained with this method. Different alignment methods used in the literature and the resulting order parameter or polarization ratio are listed in Table 1.

Table 1. Comparison of methods for aligning NRs

View Table

2. Experimental section

2.1 Synthesis of the CdSe/CdS dot-in-rods

The CdSe/CdS dot-in-rods are synthesized according to a procedure described in the literature [8] (see Appendix A for details). First, CdSe quantum dots (QDs) with a wurtzite structure and an average diameter of 2.3 nm are synthesized. On these core QDs, an anisotropic CdS shell is grown using phosphonic acids as the ligands to obtain CdSe/CdS dot-in-rods with an average diameter of 4.8 nm and an average length of 51.5 nm. Finally the CdSe/CdS NRs are dispersed in chloroform and the solution is filtered with a 0.2 µm PTFE syringe filter. For illustration, Fig. 1 shows a transmission electron microscopy (TEM) image of the NRs. These particular anisotropic structures are preferred to other types of quantum rods due to their very high photoluminescence quantum yield which is reported to be up to 75% [8] (see Appendix B for details).

 

Fig. 1 TEM micrograph of the CdSe/CdS NRs.

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2.2 Sample preparation and experimental setup

Standard optical lithography is used to pattern a 30 nm thick ITO layer on a glass substrate. The ITO electrodes in different designs have an interdigitated finger pattern, with each line 2600 μm long and 6 μm wide, with a gap of 4 or 6 μm (see Fig. 2(b)). The electric field in the gap can be approximated by assuming a homogeneous field E = V/G, with V the applied voltage and G the size of the gap. For comparison, each substrate has a second electrode finger pattern which is not connected to the voltage source during deposition.

 

Fig. 2 (a) A sketch of the experimental setup: a glass substrate with interdigitated ITO electrodes is pulled out of a colloid suspension in the presence of an electric field. (b) The width of the ITO electrodes is 6 μm and the gap is 4 or 6 μm (not to scale). (c) The electric field is perpendicular to the ITO lines, and θ is the angle between the electric field and the NR long axis.

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The experimental setup and pattern are schematically shown in Figs. 2(a) and 2(b). A function generator (TTi-TG315) and a voltage amplifier (FLC electronics-A800X) are used to apply an AC voltage of 70 V root-mean-square (rms) with a frequency of 1 kHz or 50 kHz on electrodes with gaps of 6 µm or 4 µm respectively, to a 2 × 2.6 mm pixel, resulting in electric fields of 11.8 MV/m or 17.7 MV/m respectively. For both conditions, the deposition is realized by immersing the substrate vertically in a 10 nanomolar NR solution. The AC voltage is applied and the substrate is then pulled out completely at a speed of 85.7 mm/minute. After the chloroform has dried (in a few seconds), the voltage is switched off. With these values for the NR concentration in solution and pull-up speed, a high density of nanorods on the surface is obtained without clustering. For higher pull-up speeds or lower NR concentrations in solution, the surface densities of NRs are small and the signals in the fluorescence microscope are weak.

3. Results and discussion

After deposition, the layer is imaged with a fluorescence microscope (Nikon-eclipse Ti). The NRs are excited by the UV band (330-380 nm) of a Xenon lamp. The NR emission is detected by an Andor CCD camera after passing through a long wavelength pass filter and a linear polarizer. The fluorescence microscopy images in Fig. 3 are for the substrate with ITO in absence (Fig. 3(a)) and presence (Fig. 3(b)) of an electric field (~17.7 MV/m, 50 kHz) during dip coating. In Fig. 3(a) there are (practically) no deposited NRs, the moderate brightness of the gap region is due to a weak fluorescence of the glass, whereas ITO absorbs UV light. Figure 3(b) shows that NRs are not deposited on the electrodes and that when the electric field is present, many NRs are deposited inside the gap. The comparison between Fig. 4(a) and 4(b) indicates that the NRs are attracted by the strong electric field near the surface of the substrate between the electrodes, which is well known for high-dielectric particles in an electric field gradient. The distribution of the NRs inside the gap is also observed in atomic-force microscopy (Fig. 3(c)). Both techniques reveal a higher density of rods near the center of the gap.

 

Fig. 3 Fluorescence microscopy images of a substrate (electrode gap of 4 µm indicated by white line) after dip-coating in absence (a) or presence (b) of an electric field, and AFM image of the same substrate in presence of an electric field (c).

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Fig. 4 AFM images of aligned NRs between the electrodes (a) deposited with an AC field of 11.8 MV/m (1 kHz) and (b) deposited with an AC field of 17.7 MV/m (50kHz). (c,d) Processed images to identify individual NRs. (e,f) Histograms of the azimuth angles of the NRs in images c and d.

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The orientation of the NRs depending on the deposition condition (11.8 MV/m or 17.7 MV/m) is investigated in detail by AFM (Fig. 4). Both Figs. 4(a) and 4(b) show a preferential horizontal alignment of the NRs.

In order to analyze the orientation of NRs from the image in an objective way, the image data are analyzed by a computer program. In a first step, the image is subjected to a threshold to obtain a black and white image. In the second step, white regions that are surrounded by black pixels are identified. To avoid the analysis of NR clusters and small noise related regions a maximum and minimum area for the white regions is set. Only the white regions with an area within the defined interval are ascribed to individual NRs and are represented in Figs. 4(c) and 4(d), in the cases E = 11.8 MV/m and E = 17.7 MV/m respectively. The white regions are then fitted to ellipses and the azimuth angles θ of the long axes are determined. Figures 4(e) and 4(f) finally show the histograms of the azimuth angles θ for the selected NRs, in the cases E = 11.8 MV/m and E = 17.7 MV/m respectively.

To quantify the degree of NR alignment in the images, we use the orientation order parameter S, which is defined as S = <2cos2θ-1>, with θ the angle between the NR long axis of an individual NR and the average direction of the long axes of all NRs. This expression accounts for the two-dimensional nature of the NR alignment and yields S = 1 in the case of perfect alignment and S = 0 in the case of random orientation [23, 27]. The obtained S is 0.67 for the NRs deposited in the stronger electric field at higher frequency with a density of 71 NR/µm² (Fig. 4(b)) and 0.45 for the NRs deposited in the weaker field at lower frequency with a density of 161 NR/µm² (Fig. 4(a)).

For the stronger electric field the dielectric torque is larger, which contributes to a better alignment of the nanorods with the direction of the electric field. As the electric fields that we use are quite high, we expect the dielectric torque (~E²) to be dominant over the dipolar torque (~E) which is related to the permanent dipole moment of the particles [31]. The rotational relaxation time is the time constant for returning to a random orientation after the NR has been aligned by an electric field. If the frequency of the applied voltage is higher, the time interval during which the sinusoidal electric field is close to zero will be shorter. If this time interval is shorter than the rotational relaxation time [31], then the nanorods will not have the time to obtain a random orientation. The rotational relaxation time of the rods around their short axis is about 40 µs, which corresponds to a relaxation time of 27 kHz. Therefore we can expect a higher order parameter for NRs deposited with an electric field with higher frequency.

For the sample with the highest obtained order parameter (17.7 MV/m, 50kHz), the polarization of the fluorescence of the NRs is measured with a fluorescence microscope. The fluorescence intensity is collected by an objective (60x), passes through a rotatable linear polarizer and is detected by an Andor CCD camera. For the azimuth of the polarizer θ_pol we choose the same reference as for θ, namely the direction of the electric field, which is horizontal in Fig. 2(c). We observe the strongest photoluminescence when the polarizer (and the electric field of the light that is transmitted) is parallel to the alignment of the NRs (θ = 0). This is in agreement with earlier polarization measurements on individual NRs [6] and aligned NR arrays [8, 9, 15]. Fig. 5(a) shows the fluorescence images of the NRs for different orientations of the polarizer.

 

Fig. 5 (a) Fluorescence microscopy images of the NRs located in the gap between two electrodes (dashed lines), for four azimuthal orientations of the polarizer. (b) Variation of the integrated fluorescence intensity of the NRs in the same region as a function of the azimuth of the polarizer (θ_pol).

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The average intensity I of the fluorescence is obtained by averaging the grey scale values over the pixels within a selected region in the image. Figure 5(b) shows the average intensity I as a function of θ_pol which corresponds to a function $I_{\perp }+\left(I_{\parallel }-I_{\perp }\right){\cos }^2{\theta }_{pol}$with $I_{\perp }$ and $I_{\parallel }$ the average intensities when the polarizer is oriented respectively perpendicular and parallel to the field that has been applied. The polarization ratio is expressed as Π_r = $\left(I_{\parallel }-I_{\perp }\right)/\left(I_{\parallel }+I_{\perp }\right)$ [15]. From Fig. 5(b) we find that the polarization ratio is 0.60, whereas the polarization ratio for single CdSe/CdS NR has been reported to be 0.75 [9]. If we assume that the dipole moment is parallel to the NR direction, the polarization ratio should be equal to the order parameter S. The fact that the polarization ratio (~0.60) is slightly lower than the order parameter (~0.67), may be ascribed to the fact that the transition dipole moment of the emission process is not completely parallel to the NR long axis, or may be due to measurement errors. We verified that the fluorescence from regions with randomly oriented NRs yields a vanishing polarization ratio.

4. Conclusion

We have demonstrated a method for depositing aligned NRs on a substrate, based on an applied electric field. By applying a high electric field with high frequency, we obtained an alignment with order parameter 0.67, as determined by analysis of AFM images, and a polarization ratio of 0.60, as observed by polarization fluorescence microscopy. Aligned NRs may have many applications such as polarized light emitters, polarized fluorescence and polarization-selective detectors. The deposition method is fast, facile, and can be applied on relatively large substrates. In addition, we believe that this method can be extended to the deposition or printing of all kinds of aligned anisotropic particles.

Appendix A. Synthesis of the CdSe/CdS dot-in-rods

The CdSe/CdS dot-in-rods are synthesized according to a procedure described in the literature procedure [8]. CdSe cores QDs are prepared from a mixture of 0.12 g of CdO, 6 g of trioctylphosphine oxide (TOPO) and 0.56 g of octadecylphosphonic acid (ODPA) which is degassed under vacuum at 120 °C for 1 hour. Next, the mixture is heated to 350 °C under nitrogen atmosphere and a mixture of 0.116 g of Se and 0.72 g of trioctylphosphine (TOP) is quickly injected. The reaction time is adjusted to obtain CdSe QDs with a diameter of 2.3 nm. The reaction is then quenched and the QDs are purified three times by centrifugation, using toluene and isopropanol as the solvent and the non-solvent respectively.

The CdSe/CdS dot-in-rods are prepared from a mixture of 0.057 g of CdO, 3 g of TOPO, 0.25 g of ODPA and 0.08 g of hexylphosphonic acid (HPA) which is degassed under vacuum at 120 °C for 1 hour. Next, the mixture is heated to 350 °C under nitrogen atmosphere and 1.8 ml of TOP are injected. Subsequently, at the same temperature 0.12 g of S in 1.8 ml of TOP and the above mentioned CdSe QDs are injected in the reaction mixture. The amount of CdSe cores is adjusted to obtain dot-in-rods with a diameter of 4.8 nm and a length of 51.5 nm. The reaction is quenched after 8 minutes and the dot-in-rods are purified three times by centrifugation, using toluene and isopropanol as the solvent and the non-solvent respectively.

Appendix B. Characterization techniques

Atomic force microscopy (AFM) images are recorded using a Molecular Imaging PicoPlus microscpoce working in alternative contact mode. The transmission electron microscopy (TEM) images are taken using a Cs corrected JEOL 2200 FS microscope (Fig. 6). Absorption spectrum is taken using a Perking Elmer Lambda 950 spectrometer (Fig. 7(a)).Steady-state photoluminescence measurements are performed with an Edinburgh Instruments FLSP920 setup. The emission spectrum is recorded for an excitation wavelength of 365 nm and is corrected over the sensitivity of the detector (Fig. 7(b)).

 

Fig. 6 TEM image of the CdSe/CdS dot-in-rods.

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Fig. 7 (a) Absorption spectrum of the CdSe/CdS dot-in-rods (inset: magnification on the range 500 to 600 nm highlighting the absorption feature of the CdSe cores). (b) Emission spectrum of the CdSe/CdS dot-in-rods.

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Acknowledgments

This research was supported by the Interuniversity Attraction Poles program of the Belgian Science Policy Office, under grant IAP P7-35.

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