Power Penalty

The superior EMS performance of the proposed package is also demonstrated through the measurement of the bit-error-rate (BER) tests. Figure 4.17 shows the BER versus the received optical power for three different cases (50 wt% of the MWCNTs), unpackaged module without radiated interference (case A), unpackaged module with radiated interference (case B), and packaged module with radiated interference (case C). As shown in Figure 4.14, the radiated monopole is at 3 cm distance to the tested module with 1 Vpp amplitude of the excitation. Comparing with the case A, the case B with strongly radiated noise significantly needs larger optical power to keep the same BER. As shown in Figure 4.17, the received optical power is about -11.1 dBm and -8 dBm for cases A

10 20 30 40 50 60

40 45 50 55 60

Mask Mar g in (%)

Weight Percentage of CNT (%)

Figure 4.16 The relationship between mask margin and MWCNT weight percentage at the amplitude of monopole type antenna of 0.75 Vpp.

MWCNT (wt%)

and B, respectively, to achieve the BER of 10^-12. However, for the case C with the package housing clearly shows that the BER performance is significantly improved. The optical power is about -10.6 dBm for the BER 10^-12. Comparing with the case B, the case C fabricated by the proposed MWCNT-LCP composite package significantly increases the electromagnetic immunity to the radiated interference with about 2.6 dBm optical power saving at BER 10^-12.

Figure 4.18 shows the relationship between power penalty and MWCNT weight percentage. The amplitude of interference monopole type antenna was 1 Vpp. The power penalty is defined as the received power difference between unpackaged boxes without radiated interference and packaged boxes with radiated interference in dB unit at BER equal to 10^-12. The result showed that the power penalty decreased as the MWCNT weight percentage increased. The power penalties are less than 1.5 dB for three different

-20 -15 -10 -5

10

^-4

10

^-6

10

^-8

10

^-10

10

^-12

BE R

Received Power (dBm)

Case A Case B Case C

Figure 4.17 The BER versus the received optical power for three different cases, unpackaged module without radiated interference (case A), unpackaged module with radiated interference (case B), and packaged module with radiated

interference (case C).

CNT weight percentages. The result indicates that the higher weight percentage the MWCNTs have, the better shielding ability and EMS performance are.

10 20 30 40 50 60

0.0 0.5 1.0 1.5 2.0

Power Penalt y ( d B)

Weight Percentage of CNT (%)

Figure 4.18 The relationship between power penalty and CNT weight percentage at the amplitude of monopole type antenna of 1Vp-p.

MWCNT (wt%)

4.4.5 Summary

In summary of section 4.4, The EMS performance of the 2.5 Gbps optical transmitter and receiver module packaged in this novel polymer-based MWCNT has been examined.

Under intended radiated interference, the package fabricated by MWCNT-LCP composites significantly improved the mask margin and power penalty for a 2.5 Gbps optical receiver. The mask margins were improved 46%, 53%, and 54% for 20 wt%, 30 wt%, and 50 wt% MWCNT-LCP packaged modules, respectively, under the intended inference from a monopole type antenna (0.75 Vpp amplitude). This shows that the mask margin increases as the MWCNT weight percentage increases. And the bit-error-rate test showed the proposed MWCNT-LCP composite package significantly increases the electromagnetic immunity to the intended radiated interference (monopole type antenna with 1 Vpp amplitude) with about 2.6 dBm optical power saving at BER 10^-12 by comparing the unpackaged module with radiated interference to the packaged module with radiated interference.

This study demonstrated that the plastic optical transmitter and receiver modules with the more weight percentage of the MWCNTs exhibited higher SE, and hence showed effective EMS performance, a better mask margin, and a lower power penalty. This clearly indicates that the MWCNT-LCP composites with their high SE are suitable for packaging low-cost, light-weigh, and high-performance EMS plastic transceiver modules used in FTTH lightwave transmission systems.

References

[1] P. G. Collins, P. Avouris, “Nanotubes for electronics”, Scientific American, December issue, pp. 62-69, 2000.

[2] Stand Test Method for Sheet Resistance of Thin Metallic Films with a Collinear Four-Probe Array, ASTM F390-97.

[3] Standard Testing Method for Measuring the Electromagnet Electromagnetic Shielding Effectiveness of Planner Materials, ASTM D4935-99. (ASTM, Philadelphia, PA, USA).

[4] T. L. Wu, W. S. Jou, S. G. Dai, and W. H. Cheng, “Effective electromagnetic shielding of plastic packaging in low-cost optical transceiver modules”, J.

Lightwave Technol., vol. 21, no. 6, pp. 1536–1543, Jun. 2003.

[5] P. F. Wilson, M. T. Ma, and J. W. Adams, “Technique for measuring the electromagnetic shielding effectiveness of materials: part I: Far-field source simulation”, IEEE Trans. Electromagn. Compat., vol. 30, pp. 239–247, Aug. 1988.

[6] W. S. Jou, T. L. Wu, S. K. Chiu, and W. H. Cheng, “Electromagnetic shielding of nylon-66 composites applied to laser modules”, J. Electron. Materials, vol. 30, no.

10, pp. 1287–1293, 2001.

[7] WITHDRAWN STANDARD: D4935-99 Standard Test Method for Measuring the Electromagnetic Shielding Effectiveness of Planar Materials (Withdrawn 2005).

[8] W. H. Cheng, W. C. Hung, C. H. Lee, G. L. Hwang, W. S. Jou, and T. L. Wu, “Low cost and low electromagnetic interference packaging of optical transceiver modules”, J. Lightwave Technol., vol. 22, no. 9, pp. 2177–2183, Sept. 2004.

[9] T. L.Wu, M. C. Lin, C. W. Lin, T. T. Shih, and W. H. Cheng, “High electromagnetic susceptibility performance plastic package for 10 Gbit/s optical transceiver modules”, Electron. Lett., vol. 41, no. 8, pp. 494–495, Apr. 2005.

[10] C. R. Paul, “Introduction to Electromagnetic Compatible”, A Wiley-interscience Publication, ch. 2. pp. 42-77, 1992.

[11] Limits and Methods of Measurement of Radio Interference Characteristics of Information Technology Equipment, CISPR Publication 22, 1985.

[12] H. W. Ott “ Noise Reduction Techniques in Electronic Systems”, A Wiley-interscience Publication, 1988.

[13] FCC, FCC Method of Measurement of Radio Noise Emissions from Computing Devices, FCC/OST MP-4, July, 1987.

[14] Department of Defense (US), Electromagnetic Emission and Susceptibility Requirements for the Control of Electro-Magnetic Interference, MIL-STD-461B, April 1, 1980.

[15] Electromagnetic Interference Characteristics, Measurement of, MIL-STD-462, July, 1967.

[16] J. T. DiBene, and J. L. Knighten, “Effects of device variations on the EMI potential of high speed digital integrated circuits”, IEEE Electromagn. Compat., pp. 208-212, Aug. 1997.

[17] D. M. Hockanson, X. Ye, J. L. Drewniak, T. H. Hubing, T. P. V. Doren, and R. E.

DuBroff, “FDTD and Experimental Investigation of EMI from Stacked-Card PCB Configurations”, IEEE Trans. Electromagn. Compat., vol. 43, no. 1, pp. 1-9, Feb.

2001.

CHAPTER 5

DISPERSION OF CARBON NANOTUBES

5.1 Introduction

Recently, MWCNTs have been the focus of considerable research and development for use in nanoscale electronic and optoelectronic applications, such as integrated circuit (IC) interconnections [1], optical emission devices[2], and optical transceiver modules [3-4]. However, the high aspect ratio from 200 to 1000 and low ionic character of MWCNTs make them not easily dispersed within the host matrices. The lack of dispersing ability in the polymer matrices is caused by internal van der Waals force among the MWCNTs and their consequent aggregation [5-10]. Without a fine dispersion, the MWCNTs may form local clusters and poor homogeneity existed in the MWCNT composite. Here the term “dispersion” means the dispersive and homogenized distribution of the guest material inside the host matrix, it is different from the frequency dependent chromatic dispersion.

MWCNTs usually have a strong tendency for aggregation and difficulty to disperse in either water or any organic solvents due to the inherent van der Waals attractions among MWCNTs [5-10]. To avoid MWCNTs self-aggregation and to reduce inorganic-organic network adhesions are two key issues to improve reinforcement effects of the MWCNTs. It has been reported that the carbon nanotubes can be dispersed by the methods of chemical modification or physical adhesions [11-13]. The chemical modification requires a treatment of acids, such as HNO3, and H2SO4, which may adversely damage the MWCNT structure and also may have a great deal of surfactant addition in the adhesion process [11]. The physical method of ionic liquid (IL) dispersants [12-13] has been used to disperse the MWCNTs. The method involves an intensive grinding of the mixture of IL and MWCNTs and follows by dissolving into N-methylpyrrolidone (NMP) solvent. The van der Waals force between MWCNTs was mitigated by mixing the imidazolium ions in the MWCNT-IL hybrid solution [12]. The dispersion is efficiently reinforced by taking advantage of a strong affinity of the imidazolium cation ion toward the π-electronic MWCNT surface.

Despite numerous studies on physical method of MWCNT dispersion using the IL dispersant [12-13], detailed dispersion mechanisms of IL in MWCNTs for

understanding and fine dispersion of MWCNTs in the polymer matrices are not available. In this study, the dispersion mechanism of MWCNTs using IL is qualitatively presented. A novel polyimide film material, consisting of finely dispersed MWCNTs, is demonstrated for use in packaging optical transmitter and receiver module. We have found that the ionic charge force of the cation-π interaction and interaction force between alkyl groups in the IL-MWCNT hybrid solution are the dominant mechanisms to disperse the aggregation of MWCNTs. The organic cations of the IL interact with π-electronic compounds through the so-called cation-π interaction [12] as an initial step. The cation-π interaction anchors the cation ions of IL on the MWCNT surface. Then a steric isolation effect from the hydrophobic force between alkyl groups of IL molecules makes the dispersion. Another hydrophobic force between alkyl groups of IL and polyimide (PI) matrices also offers the effective intermolecular force to sustain the dispersion, while mixing the MWCNT-IL hybrid solution with polyimide precursor. For developing a cost effective material, a fine dispersion of MWCNTs in the polymer matrices and association with the dispersion mechanism understanding is essential.

5.1.1 Aggregation of Carbon Nanotube

The aggregation of MWCNTs is mainly from the van der Waals forces among MWCNTs. The van der Waals forces among MWCNTs can be modeled as the forces between electrical multipoles. The single carbon nanotube is constructed by a hexagonal ring structure of carbon atoms [14-15]. For each individual carbon hexagonal ring (similar as benzene), there are six carbon atoms connected by σ-bond (SP² orbit). There is a free π electron (2P orbit) for each carbon. It can form a π bond with the adjacent 2P orbits. In a hexagonal structure, the free electrons are not enough to form 6 π bonds. They can share free electrons, which is called a conjugate π system, also called delocalized electron cloud. The π electrons and the hexagonal ring center of carbon atoms can form a transient dipole.

There is a high chance that the electron density will not be evenly distributed throughout a nonpolar molecule like a multiwall carbon nanotube. When an uneven distribution occurs, the temporary multipoles are created. This multipole may interact with other nearby multipole via the π-π interaction force since it is a conjugated-π

electronic structure of carbon nanotubes. The π-π interaction forces between the adjacent nanotubes result in aggregated nanotubes with the van der Waals potential energy of approximate 0.5 eV per nanometer of two parallel nanotubes [16]. London forces is one of the van der Waals force, which is used to describe the weak intermolecular forces that arise from the attractive force between transient dipoles (or multipoles). The van der Waals force between MWCNTs may be modeled as the London force since the electron distribution upon the MWCNT surface is not even, they are dense in each hexagonal rings.

The surface of MWCNT can be described by a lot of transient dipoles spread upon it.

However, the whole carbon nanotube is neutral electricity and not a polar tube. Once the distance between MWCNT and MWCNT is close, the weak van der Waals force can make the MWCNTs aggregated. Van der Waals force is the main force contributed to the MWCNT aggregation. Van der Waals force consists of two parts; one is the repulsive force and another is the attractive force. The repulsive force usually can be expressed by the Pauli Exclusion Principle [17]. For the attractive force, it consists of dipole-dipole interaction force, electro-static force (van der Waals-Keesom), permanent multipole-induced multidipole interaction force (van der Waals-Debye), and transient dipole-induced dipole interaction force (van der Waals-London) [18].

A simple van der Waals model can be mathematically expressed by Lennard-Jones potential equation [19]. Neutral atoms and molecules are subjected to two distinct forces in the limit of large distance and short distance. There two forces are an attractive force at long ranges and a repulsive force at short ranges. The van der Waals force modeled by Lennard-Jones potential equation is expressed as [19]

⎥

where the ULJ is the potential energy, the F is the inter-particle force, ε is the depth of the potential well, and the σ is the (finite) distance at which the inter-particle potential is zero,

and the R is the inter-particle distance. The Lennard-Jones potential equation is an approximation to represents the van der Waals force behavior.

The potential energy per unit length between two identical and parallel carbon nanotubes can be expressed as U

( )

R [16], perpendicular distance between tube centers, the r is the tube radius. Both variables θ ₁ and θ are in the integrals range from 0 to 2₂ π . A and B are the attractive and repulsive constants in the Lennard-Jones potential.

5.1.2 Dispersion Model

The ionic liquid (IL) dispersant is used to disperse MWCNTs. The cation- π interaction and the hydrophobic force of the alkyl groups in the IL-MWCNT hybrid solution interaction are the dominant forces. The IL is a kind of electrolyte composed of two ions, the cation and anion. An IL of 1-hexadecyl-3-methylimidazolium chloride (HDMIC)in useis chemically expressed in Figure 5.1. Usually, the cation part of IL is a kind of imidazolium, which attached with a long alkyl group.

Figure 5.1 The chemical expression of HDMIC ionic liquid dispersant.

The force between the cations and π electrons of MWCNTs is named as cation-π interaction force [12]. It is a kind of electro-static force or ionic charge force. By cation-π interaction, the cations are attached on the surface of MWCNTs. Figure 5.2 shows the aggregated MWCNTs are mixed with IL then dissolved into the N-methylpyrrolidone (NMP) solvent.

Figure 5.2 (a) Schematic diagram of aggregated MWCNTs, (b) ionic liquid (IL), and (c) aggregated MWCNTs with IL in NMP solvent.

MWCNT

MWCNT:

MWCNT

MWCNT:

IL

+

Cation ion of IL:

Alkyl group of IL:

IL

+

Cation ion of IL:

Alkyl group of IL:

IL

++

Cation ion of IL:

Alkyl group of IL:

(a) (b)

NMP (c)

The dispersion phenomenon after well mixing IL by stirring the hybrid solution. The cation ions of IL are uniformly attached on the MWCNTs, and the hydrophobic forces between the alkyl groups of IL make the dispersion via the steric isolation effect, as shown in Figure 5.3 (a). The dispersion effect increases as the IL ratio increases, as shown in Figure 5.3 (b).

The alkyl group is a long-chain hydrocarbon attached on the imidazolium cation.

There are forces between an alkyl group and other alkyl groups due to their hydrophobic attraction property. These hydrophobic forces contribute to the dispersion of MWCNTs since their steric isolation effect. The alkyl groups of IL will also interact with the non-polar polyimide molecules, while a matrix material mixed together, such as the polyimide matrix.

The cation-π interaction force is a kind of ionic charge force. It can be expressed in the Coulomb electrostatic force [20], formula as

( )

Figure 5.3 (a) Schematic diagram of MWCNTs dispersed by IL, (b) well-dispersion MWCNTs by IL.

₂ charges with coulomb unit, the ε is the dielectric constant in vacuum, R is the distance ⁰ between cation and π electron. The total interaction force between cations and π electrons can be expressed as

The interaction force between two alkyl groups is the gradient of potential energy, and is named as hydrophobic force (Fh). There are similar studies for the hydrophobic force. For example, Christenson and Claesson [21] observed from their experiments that the hydrophobic forces measured between Langmuir-Blodgett (L-B) deposited monolayers of long-chain hydrocarbon and fluorocarbon surfactants on mica clearly followed a double-exponential force law [21]

Fh/R = C1exp(-H/D1) + C2exp(-H/D2) (5-7)

where the R is the mean radius of curvature of the interacting bodies, and the H is the closest separation between the two curved surfaces. The C₁ and D₁ represent the short-range part of the interaction and the C2 and D2 the long-range part. The C1 and C2 are pre-exponential factors. The D1 and D2 are known as the decay length.

Another example, surface-tension model can be used to calculate the hydrophobic force between two hydrocarbon molecules such as force between two methane molecules in water [22].

where G∆ is the corresponding free energy change, σ is surface-tension parameter, R is the separate distance between two methane molecules, ∆ is the change in the solvent-A accessible area. f is the hydrophobic force.

Before adding IL dispersant, the binding energy of van der Waals force between a hexagonal ring on a MWCNT and another hexagonal ring on neighboring MWCNT is the order of 1 kcal/mol [16]. After adding IL dispersant, they have three major interaction forces involved. The predominant is the cation-π interaction which has the binding energy of the order 9.4 kcal/mol [20], the minor binding energy is contributed from the hydrophobic interaction including the binding energy between alkyl groups and the binding energy between alkyl groups and matrix molecules.

Cation-π interaction belongs to ionic charge force with a binding energy greater than that of weak van der Waals force. Besides, according to equation (5-1), the van der Waals potential energy is approximately proportional to 1₆

R for the attraction. And the van der Waals force is in a relative small scale in comparison to the other forces, such as cation-π and hydrophobic forces. For example, the cation-π binding energy between the organic ion of NMe4+

and benzene in equation (5-4) is around 9.4 kcal/mol [20] and the binding energy of hydrophobic interaction between two methanes is around 2.7 kcal/mol [21].

However, the binding energy of the weak van der Waals force in equations (5-1) is usually less than 1 kcal/mol [16]. Table 5.1 lists the binding energies of the van der Waals interaction,the ionic charge interaction, the alkyl group interaction, and the alkyl and matrix interaction. Overall, the predominant binding energy of cation-π interaction is greater than the binding energy of aggregated MWCNTs. Ultimately, a fine dispersion of MWCNTs in the matrix in the presence of IL occurs.

Table 5.1 Binding energies of van der Waals force, cation- π interaction, alkyl-alkyl interaction, and alkyl-polyimide interaction.

Type

Energy Without IL

Binding energy van der Waals cation-π alkyl-alkyl alkyl-PI Binding energy (kcal/mol) 1 9.4 2.7 2.7

Reference [16] [20] [22] [22]

Dispersion with IL

5.2 Dispersion Measurement

5.2.1 Raman Spectroscopy Investigation

To identify the characteristics of MWCNT before and after adding IL dispersants, the dispersed MWCNT-PI composite is investigated under the Raman spectrometer. The intensity peak locations of the non-dispersed MWCNT-PI composite and the dispersed MWCNT-PI composite in the Raman spectrum are compared. The Raman spectrum is shown in Figure 5.4. The Raman shift of dispersed MWCNT-PI has two peaks at 1350 cm^-1 and 1580 cm^-1, which are essentially identical to the peak locations of non-dispersed MWCNT-PI. The peak width and the relative intensity between two peaks of dispersed are similar to those of the non-dispersed. This indicates the dispersion is mostly like a physical interaction rather than a chemical reaction, and the MWCNT-PI are almost not functionalized after IL dispersion process.

Figure 5.4 Raman spectroscopy shows peaks of IL dispersed MWCNT-PI at 1580 cm^-1 and 1350 cm^-1, which are essentially identical to the non-dispersed MWCNT-PI.

5.2.2 Uniformity Investigation by UV-vis Spectrometer

An UV-vis spectrometer is used to measure the uniformity of the MWCNT-IL hybrid solution at the absorption of 550 nm light. The absorption is defined as

⎟⎟⎠

⎜⎜ ⎞

⎝

− ⎛

i t

P

log₁₀ P , where the P_i is intensity of incident light, and the P_t is the intensity of

transmitted light. A higher absorption value means the MWCNTs are more dispersed and then more uniform in the solution. Figure 5.5 shows the absorption of various weight ratios of IL dispersed MWCNT hybrid solution, the variation of IL content from 0.1 to 0.5 indicates a dispersion effect is reinforced as the IL content increases.

Figure 5.5 UV-vis spectrometer absorption of various weight ratios of MWCNT-IL hybrid dispersed in NMP solvent at wavelength of 550 nm.

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

0 0.1 0.2 0.3 0.4 0.5 0.6

IL/MWCNT

Absorption

5.2.3 Percolation Phenomenon of Electrical Conductivity

According to the basic EM shielding theory, the SE increases as the material conductivity increases [3-4]. The more MWCNT material is added, the more overlapping conductive MWCNT networking, and hence the higher conductivity and the higher SE are expected. However, the IL-dispersed MWCNT composites offer a high electrical

According to the basic EM shielding theory, the SE increases as the material conductivity increases [3-4]. The more MWCNT material is added, the more overlapping conductive MWCNT networking, and hence the higher conductivity and the higher SE are expected. However, the IL-dispersed MWCNT composites offer a high electrical

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