Our group developed a high-performance unipolar charger for sub-10-nm particles using a surface-discharge microplasma by increasing the charging time and minimizing the electrostatic deposition loss of the charged particles. An investigation of the discharge voltages of various discharge voltage waveforms demonstrated that a sinc function of time, t, that is, (sin ωt)/ωt with a bias voltage, achieved a high extrinsic charging efficiency (a high yield of charged particles) by generating a high concentration of ions and suppressing the electrostatic deposition of the charged particles. In trial operation at the optimal discharge voltage and an aerosol flow rate of 2.5 L/min, the charger attained intrinsic and extrinsic charging efficiencies of 79.3 and 61.4% for 10-nm particles and intrinsic and extrinsic charging efficiencies of 48.0 and 34.6% for 5-nm particles, respectively.

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