How to create .xlsx file for Excel version 2010 in c#?

This is my code again. Using this code i have .xls file which can store up to 65k rows, but i need more. I know, that Excel file from version 2007 can store 1 mln rows. Do I need to add special library to save file in .xlsx or what?

//------------------Create Excel App--------------------
Excel.Application xlApp;
Excel.Workbook xlWorkBook;
Excel.Worksheet xlWorkSheet;
object misValue = System.Reflection.Missing.Value;

xlApp = new Excel.Application();
xlWorkBook = xlApp.Workbooks.Add(misValue);
xlWorkSheet = (Excel.Worksheet)xlWorkBook.Worksheets.get_Item(numberOfLetters);

     foreach (string letter in letters)
     {
      xlWorkSheet.Cells[rowIndex, 1] = letter; 
      rowIndex++;
     }

xlWorkBook.SaveAs(pathXL, Excel.XlFileFormat.xlWorkbookNormal, misValue, misValue, misValue, misValue, Excel.XlSaveAsAccessMode.xlExclusive, misValue, misValue, misValue, misValue, misValue);
xlWorkBook.Close(true, misValue, misValue);
xlApp.Quit();